AIR  NAVIGATION 

FOR   FLIGHT   OFFICERS 


AIR    NAVIGATION 
FOR   FLIGHT  OFFICERS 


BY 
LlEUT.-COMMANDER    A.    E.    DIXIE,    R.N. 


NEW    YORK 
D.   APPLETON   ftf  COMPANY 

1917 


-.'*..  on, 


PREFACE 

THIS  work  has  been  undertaken  in  the  hope 
that  it  may  prove  of  assistance  to  officers  in 
the  Royal  Naval  Air  Service,  as  it  condenses 
into  a  small  compass  all  the  subjects  in 
navigation  they  are  required  to  know. 

A.  E.  DIXIE. 

NAVIGATION  SCHOOL,  PORTSMOUTH. 
March,  1917. 


O  O  >«y  r*  rv  n 


CONTENTS 

CHAPTER  I 

PAGE 

Elementary  magnetism — The  earth's  effect  on  a 
compass  needle — Definitions — Various  methods 
of  making  magnets — Alloy  used  in  making  mag- 
nets— Effect  of  temperature  on  magnets — Effect 
of  magnetism  on  hard  and  soft  iron — Permanent 
magnetism  and  its  effect — Sub-permanent  mag- 
netism, its  cause  and  effect — The  effect  of  a 
magnet '  end  on  '  and  '  broadside  on  '  .  .  i 

CHAPTER  II 

The  magnetic  compass — Liquid  used  in  a  compass — 
To  remove  a  bubble  from  a  compass — Remarks  on 
placing  a  compass — Features  essential  in  an  aero- 
plane compass — Magnet  block — The  effect  of 
'  banking  '  on  a  compass  .....  23 

CHAPTER  III 

The  analysis  and  adjustment  of  deviation — The 
various  coefficients — Analysis  of  a  table  of  devia- 
tions   32 

CHAPTER  IV 

The  practical  correction  of  a  compass — Methods  of 
swinging — Marking  out  a  flying  ground  for 

swinging .43 

vii 


CONTENTS 
CHAPTER  V 

TAGE 

Correcting  courses — Naming  deviation — Rules  for 
getting  the  correct  bearing  from  the  bearing 
tables — Final  notes  .  .  .  .  -57 

CHAPTER  VI 

Meteorology — Types  of  weather — Cloud  formation — 
Fog  formation — Synoptic  chart — Dunboyne's 
weather  report  ......  67 

CHAPTER  VII 

General  weather  in  the  British  Islands — Storm  signals 

— Beaufort's  system  of  weather  notation    .          .       80 

CHAPTER  VIII 
Forecasting  by  solitary  observer       .  86 

CHAPTER  IX 

Astronomy — Sketches  of  the  important  constellations 
— Notes  on  time — How  to  find  the  time  of  sun- 
rise and  sunset,  and  moonrise  and  moonset,  and 
the  true  bearing  at  each — Explanation  of  the 
various  tables  ......  93 

CHAPTER  X 

Admiralty   charts — Theory  and  construction  of  the 
gnomonic   chart — Theory    and    construction    of 
the  Mercator's  chart — How  to  lay  off  a  course — 
To  measure  distance         .          .          .          .          .143 
viii 


CONTENTS 

CHAPTER  XI 

To  construct  a  scale  of  longitude  for  any  plan — Con- 
ventional markings  on  a  chart — Use  of  celluloid 
protractor.  To  allow  for  drift  due  to  wind — 
Course  necessary  to  steer  to  intercept  hostile 
aircraft — Lights,  where  to  find  particulars — 
Description  of  the  various  kinds  .  .  .152 

CHAPTER  XII 
Fixing  positions     .          .          .          .          .          .          .177 

CHAPTER  XIII 

Ordnance    maps — Conventional    markings — Use    of 

squared  maps !86 

APPENDIX        ........     xg^ 

INDEX 2I3 


For  List  of  Illustrations,  &c.,  see  following  page. 


IX 


ILLUSTRATIONS    AND 
DIAGRAMS 


PAGE 


MAGNETISM .3 

COEFFICIENTS         .          .          .          ...  34  and  193 

MARKING  OUT  A  FLYING  GROUND.          ...       52 

DEVIATION    ........       60 

NOTES  ON  TRUE  BEARINGS    .....       63 

METEOROLOGY       .......       70 

CLOUDS          ......  facing      75 

WEATHER 88 

ASTRONOMY  ........       96 

TIME    .........     114 

CHART  .........     143 

CHART  SYMBOLS    .......     154 

SCALE  OF  LONGITUDE 166 

ALLOWING  FOR  DRIFT  DUE  TO  WIND      .          .          .169 
INTERCEPTING  HOSTILE  AIRCRAFT  .          .          .          -171 
FIXING  POSITIONS          .          .          .          .          .          .178 

MARKINGS  ON  ORDNANCE  MAPS      .          .          .          .189 

COEFFICIENT  E       .....  facing     193 

BAROMETER  SKETCHES  ......     196 

DEFLECTION  OF  WIND  DUE  TO  EARTH'S  ROTATION.     201 
VARIOUS  FORMS  OF  ISOBARS  ....       202-204 

HIGH  AND  Low  PRESSURE  AREAS  ....     209 

TRIANGLE  OF  VELOCITIES        .          .          .          .          .210 

Example  on    .          .          .          .          .          .          .211 


NOTE 

In  the  diagrams  throughout  the  book,  solid  black 
indicates  the  red  or  north  seeking  end,  and  broken  lines 
the  blue  or  south  seeking  end  of  the  magnet. 


Attention  is  directed  to  the   Catalogue   of  Standard  Naval 

Publications  at  the  end  of  this  book. 

x 


AIR    NAVIGATION 

FOR 

FLIGHT    OFFICERS 

CHAPTER  I 
MAGNETISM 

A  KNOWLEDGE  of  magnetism  is  absolutely 
essential  in  order  to  understand  the  action  of 
the  iron  and  steel  used  in  construction  on 
a  compass.  Also  to  know  what  causes  the 
error,  and  why  this  error  is  introduced. 

Magnetism  is  a  force  existent  all  over  the 
world,  whose  nature  is  that  it  exerts  its 
influence  on  iron  and  steel,  causing  them  to 
become  magnetic.  It  was  first  discovered  in 
a  substance  called  '  Lodestone,'  and  after- 
wards in  certain  other  iron  ores  found  in 
various  parts  of  the  world. 

These  iron  ores  are  known  as    '  natural 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

magnets/  which  are  never  used  in  compass 
adjustment,  one  reason  being  that  they  vary 
greatly  in  strength. 

Artificial  Magnets. — These  are  pieces  of 
iron  or  steel  to  which  magnetic  properties 
have  been  imparted  by  various  methods. 

They  have  the  same  magnetic  properties 
as  natural  magnets,  but  with  increased  power, 
depending  on  the  amount  of  magnetism  they 
receive. 

Any  part  of  a  magnet  contains  more  or 
less  magnetism,  but  its  greatest  power  is  con- 
centrated at  two  points  near  each  extremity, 
these  positions  being  known  as  the  '  Poles  ' 
of  the  magnet.  The  earth  itself  possesses  the 
properties  of  a  huge  magnet,  following  the 
same  laws  that  an  ordinary  magnet  does. 
Its  poles  do  not  coincide  with  the  geographical 
poles  of  the  earth,  bat  are  some  distance  from 
either ;  one  being  situated  north-west  of 
Hudson  Bay,  and  the  other  in  South  Victoria 
Land. 

They  are  not  points  like  the  geographical 
poles,  but  are  areas  of  considerable  extent. 

The  earth  being  a  magnet  has  certain 
lines  of  force  (see  Fig.  i)  passing  through 
it,  and  if  any  iron  or  steel  is  placed  in  these 

2 


MAGNETISM 

lines  of  force  they  will  be  affected  by  it  and 
become  magnetic  themselves. 

The  magnetism  in  any  magnet  is  of  equal 
and  opposite  character  at  either  pole,  and  it 
has  been  found  by  experiment  that  if  the  same 
like  named  poles  of  two  magnets  be  brought 


FIG.  i. 


into  each  other's  field,  that  they  will  repel 
one  another,  but  that  unlike  poles  will  attract 
each  other.  Hence  the  following  rule  holds 
good,  which  is  known  as  '  The  First  Law  of 
Magnetism ' : 

Like  poles  repel.     Unlike  poles  attract. 

This  rule  should  be  carefully  memorised  ; 
by  doing  so,  compass  adjustment  with  regard 

3 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 

to  the  placing  of  the  adjusting  magnet  will 
be  quite  easy  to  understand. 

Fig.  i  shows  an  ordinary  bar  magnet  with 
the  lines  of  force  emanating  from  it.  It 
has  been  found  convenient  to  imagine  the 
lines  of  force  as  issuing  from  the  north-seeking 
end,  and  entering  the  south-seeking  end. 


FIG.  2. 

Fig.  2  will  show  what  would  happen  to 
a  small  freely  suspended  magnet  if  passed 
along  an  ordinary  bar  magnet. 

As  the  earth  is  a  large  magnet  the  following 
figure  (Fig.  3)  shows  what  would  happen  to  a 
freely  suspended  magnetised  needle  if  carried 
from  one  pole  to  another. 

The  portions  of  the  magnetic  poles  visible 
are  indicated  by  two  white  semicircles. 

From  Fig.  3  it  will  be  seen  that  on  the 
line  joining  the  red  and  blue  magnetism  of 
the  earth  the  small  magnet  will  assume  a 
horizontal  position,  whilst  at  the  magnetic 
poles  it  will  be  vertical,  so  that  in  any  inter- 

4 


MAGNETISM 

mediate  place  it  will  tend  to  set  at  different 
angles  to  the  horizontal. 

This  angle  is  known  as  the  '  Dip/  and  an 
explanation  will  be  given  later. 


FIG.  3. 

A  magnet  cannot  exist  without  having 
two  consequent  poles,  one  at  each  end, 
consequently  if  it  be  divided  into  two  or  any 
number  of  pieces,  each  of  these  pieces  becomes 
a  complete  magnet  in  itself,  as  shown  in 
Fig.  4. 

5 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

In  connection  with  the  foregoing  figures 
it  will  be  noticed  that  the  magnets  are 
represented  as  red  and  blue :  red  for  the 
north- seeking  end,  and  blue  for  the  south- 
seeking  end. 

This  is  the  conventional  way  that  magnets 
are  painted,  and  from  now  onwards  the  north- 
seeking  or  red  end  of  a  magnet  or  compass 


FIG.  4. 

needle  will  be  called  the  north  pole,  and  the 
south-seeking  or  blue  end  the  south  pole. 

Hence  the  northern  part  of  the  earth 
must  be  coloured  blue,  and  the  southern 
half  red,  to  conform  with  the  law  given 
before. 

As  the  geographical  and  magnetic  poles 
do  not  coincide,  the  compass  needle  cannot, 
except  in  certain  positions,  point  to  the  true 
north,  but  at  an  angle  to  it,  according  to  the 
needle's  position  on  the  earth's  surface.  This 
angle,  which  may  have  any  value  up  to  180°, 

6 


MAGNETISM 

is  the  angle  between  the  true  and  magnetic 
meridians,  and  is  known  as  the  'Variation/ 

It  is  called  easterly  if  the  north  end  of 
the  needle  is  drawn  to  the  right  of  the  true 
meridian,  and  westerly  if  drawn  to  the  left. 

At  those  places  where  the  true  and 
magnetic  meridians  do  coincide  the  variation 
is  nothing. 

The  value  of  the  variation  has  been  found 
for  practically  all  over  the  world,  and  if 
required  it  can  be  taken  from  the  Admiralty 
Variation  Chart  or  Compass  Manual. 

The  continuous  lines  on  the  chart  denote 
that  the  variation  is  westerly;  the  pecked 
lines,  that  the  variation  is  easterly ;  and  the 
two  side  by  side  show  the  lines  of  no  variation. 

This  variation  undergoes  an  annual  change, 
probably  due  to  the  magnetic  poles  shifting. 

This  change  is  given  on  the  variation 
chart  and  also  on  Admiralty  charts,  but  for 
ordnance  maps  it  must  be  taken  from  the 
former  if  no  Admiralty  chart  is  available. 

The  magnetic  poles  are  not  points  like 
the  geographical  poles ;  that  is  to  say,  they  are 
areas  of  considerable  extent. 

The  following  definitions  will  be  found 
useful,  and  should  be  committed  to  memory 
and  thoroughly  understood. 

7 


AIR   NAVIGATION   FOR   FLIGHT  OFFICERS 

Line  of  Total  Force. — Is  the  direction  that 
freely  suspended  magnetic  needle  will  take  up 
when  under  the  influence  of  the  earth's  forces. 

Magnetic  Poles. — Are  the  two  places  on  the 
earth's  surface  where  the  total  force  is  vertical, 
and  to  which  the  needle  points  in  all  adjoining 
regions. 

Magnetic  Equator. — Is  the  line  separating 
the  red  and  blue  magnetism  of  the  earth,  and 
along  which  the  line  of  total  force  is  horizontal. 

It  does  not  coincide  with  the  geographical 
equator,  and  only  intersects  it  in  two  places. 

Magnetic  Meridian. — Is  the  vertical  plane 
passing  through  the  longitudinal  axis  of  a 
freely  suspended  magnetic  needle  when  resting 
in  a  line  of  total  force  and  free  from  local 
attraction. 

Variation. — Is  the  horizontal  angle  be- 
tween the  true  and  magnetic  meridians. 

Deviation. — Is  the  horizontal  angle  be- 
tween the  magnetic  meridian  and  the  vertical 
plane  passing  through  the  longitudinal  axis 
of  a  magnetised  needle  when  under  the 
influence  of  local  attraction. 

8 


MAGNETISM 


It  is  called  easterly  or  +  when  the  north 
end  of  the  needle  is  drawn  to  the  right  of  the 
magnetic  meridian,  westerly  or  —  if  drawn  to 
the  left  of  the  magnetic  meridian. 


Compass  Error. — Is  the  algebraical  sum  of 
the  variation  and  deviation. 

Dip. — Is  the  vertical  angle  between  the 
direction  of  a  freely  suspended  magnetic 
needle  resting  in  a  line  of  total  force  and  the 
horizontal  plane  passing  through  the  centre 
of  the  needle. 

Poles  of  a  Magnet. — Are  the  two  points 
of  maximum  intensity  situated  about  one- 
twelfth  of  the  total  length  of  the  magnet  from 
either  extremity. 

Magnetic  Latitude. — Is  measured  north  or 
south  from  the  magnetic  equator,  and  is  some- 
what similar  to  terrestrial  latitude. 

Lines  of  equal  dip  correspond  to  magnetic 
latitude. 

Horizontal  Force. — Is  the  horizontal  com- 
ponent of  the  earth's  magnetism. 

9 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

Vertical  Force. — Is  the  vertical  component 
of  the  earth's  magnetism. 

N.  B. — The  size  of  the  angle  of  dip  depends 
on  the  value  of  these  two. 

The  following  figure  shows  the  connection 
between  horizontal  force,  vertical  force,  total 
force,  and  dip. 


FIG.  5. 

H  represents  the  horizontal  force. 
Z  represents  the  vertical  force. 
T  represents  the  total  force. 
£  represents  the  angle  of  dip. 

2 

Then  ~  =  Tangent  Dip. 

rl 

.      Vert.  Force 

i.e.  77 7; =  Tangent  Dip. 

Hor.  Force 

10 


MAGNETISM 

H2  -f  Z2  =  T2 
i.e.  (Hor.  Force)2  +  (Vert.  Force)2  = 

(Total  Force).2 

Z  =  H  Tan  6 
i.e.  Vert.  Force  =  Hor.  Force  x  Tangent  Dip. 

H  =  Z  Cot  e 
i.e.  Hor.  Force = Vert.  Force  x  Cotangent  Dip. 

Z  =  T  Sin  0 
i.e.  Vert.  Force  =  Total  Force  x  Sine  Dip. 

H  =  T  Cos  0 
i.e.  Hor.  Force  =  Total  Force  x  Cosine  Dip. 

The  Methods  of  Making  Magnets.— There 
are  four  different  ways  of  making  magnets,  as 
follows  : 

(1)  By  Percussion. 

The  bar  to  be  magnetised  is  placed  in  the 
direction  of  the  lines  of  force  of  the  earth,  and 
one  end  is  smartly  tapped  with  a  hammer. 
This  induces  magnetism  in  it,  the  amount 
received  depending  on  the  number  and  force 
of  the  blows  and  the  coerrive  force  of  the  metal 
to  be  magnetised. 

The  pole  of  the  magnet  which  is  lowest  will 
be  of  opposite  polarity  to  the  hemisphere 
where  it  was  manufactured, 

(2)  By  Single  Touch. 

ii 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 

The  bar  to  be  magnetised  is  placed  on  a 
flat  surface,  and  one  end  of  a  magnet  placed 
on  one  extremity  of  the  bar  and  drawn  smartly 
along  the  length  of  the  latter,  being  lifted  off 


Result 
FIG.  6/ 

at  the  end  of  the  stroke  and  replaced  on  the 
starting-point. 

This  operation  ma}'  be  repeated  as  often  as 
necessary. 

The  end  of  the  bar  last  touched  by  the 
magnet  will  be  of  opposite  polarity  to  the  end 
of  the  inducing  magnet  touching  it. 

(3)  By  Divided  Touch. 

12 


MAGNETISM 

The  bar  to  be  magnetised  is  placed  on  a 
flat  surface,  and  the  opposite  ends  of  two 
magnets  placed  on  its  centre  and  drawn 
smartly  outwards  towards  their  respective 
ends.  This  operation  is  repeated  as  often  as 


Result 
FIG.  7. 

necessary.  The  ends  of  the  bar  last  touched 
by  the  magnets  will  have  opposite  polarity 
to  the  ends  of  the  respective  magnets  used. 

(4)  By  Electro  Magnet. 

The  bar  to  be  magnetised  is  placed  across 
the  poles  of  an  electric  magnet  and  kept 
there  as  long  as  necessary.  The  ends  of 
the  bar  will  acquire  opposite  polarity  to  the 
poles  of  the  electro  magnet. 

13 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

This  method  is  always  employed  in  the 
manufacture  of  magnets  used  in  compass  work, 
as  by  its  means  they  can  be  made  stronger  and 
more  uniform  in  power. 

Alloy  Used  in  Making  Magnets. — Magnets 


Result 
FIG.  8. 


are  made  of  hard  steel  with  a  mixture  of 
5  per  cent,  of  tungsten. 

This  has  been  found  to  increase  its  coercive 
force.  By  '  coercive  force  '  is  meant  the 
property  by  which  iron  or  steel  not  only 
retains  its  magnetism  after  it  has  been 
imparted  to  it,  but  also  the  resistance  it  puts 
up  against  being  magnetised. 

14 


MAGNETISM 

Compass  magnets,  if  properly  stowed,  i.e. 
(unlike  poles  together)  and  well  looked  after 
retain  their  magnetism  without  appreciable 
loss  for  years. 

Effect  of  Temperature  on  Magnets. — Ordin- 
ary atmospheric  changes  of  temperature  have 
practically  no  effect  on  a  permanent  magnet, 
such  as  those  used  for  compass  adjustments. 

If,  however,  it  be  placed  in  a  very  strong 
magnetic  field  of  opposite  power,  or  if  heated 
to  a  dull  red  heat,  i.e.  between  1300°  and  1500° 
Fahrenheit,  it  becomes  de-magnetised. 

On  the  other  hand,  soft  iron  increases  its 
capacity  for  receiving  magnetism  on  being 
heated,  this  increases  up  to  a  temperature  of 
1427°  Fahr.,  but  after  this  there  is  a  rapid 
decrease,  and  at  1445°  the  iron  becomes  non- 
magnetic. 

Effect  of  Magnetism  on  Hard  and  Soft  Iron. 

— The  iron  or  steel  used  in  construction  varies 
in  its  magnetic  character. 

This  necessitates  a  little  explanation. 

Iron  or  steel  may  be  classed  under  two 
headings  :  '  Hard  '  and  '  Soft/ 

Hard  iron,  on  account  of  its  coercive  force, 
does  not  pick  up  or  part  with  its  magnetism 
freely. 

15 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

It  acquires  magnetic  properties  during  its 
manufacture  on  account  of  the  hammering 
and  violence  it  has  been  subjected  to.  After 
manufacture  it  loses  some  of  this  magnetism, 
but  soon  settles  down,  and  the  residue  may  be 
regarded  as  permanent. 

Soft  iron  has  little  or  no  coercive  force, 
and  picks  up  and  loses  its  magnetism  freely, 
so  that  for  every  direction  of  the  machine's 
head  a  different  amount  of  magnetism  is 
induced. 

Soft  iron  is  seldom  absolutely  pure,  conse- 
quently it  nearly  always  retains  a  certain 
amount  of  magnetism,  not  due  to  the  lines  of 
force  of  the  earth. 

The  deviations  caused  by  hard  iron  are 
called  semicircular,  because  they  only  change 
their  sign  once  in  the  whole  circle. 

They  are  corrected  by  horizontal  magnets 
placed  longitudinally  and  transversely. 

Those  caused  by  soft  iron  are  termed 
'  quadrant al,'  because  they  change  their  sign 
in  each  quadrant. 

They  are  corrected  by  soft  iron  balls  or 
spheres  placed  on  each  side  of  the  compass. 

Sub -Permanent  Magnetism,  its  Cause  and 
Effect. — This  is  caused  by  iron  which  does 

16 


MAGNETISM 

not  come  under  the  category  of  hard  or  soft, 
but  lying  between  the  two.  After  being  on 
one  course  for  some  time  it  acquires  a  mag- 


FIG.  9. 


netic  character  due  to  the  lines  of  force  of  the 
earth,  and  this  is  accentuated  by  vibration 
of  engines,  gunfire,  etc. 

On  alteration  of  course  this  magnetism 
does  not  immediately  disappear  as  in  the  case 

17  c 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 

of  soft  iron,  but  only  does  so  gradually,  the 
time  taken  depending  on  the  length  of  time 
on  the  course  and  the  coercive  force  of  the 
metal.  It  cannot  be  corrected,  and  its  amount 
can  only  be  ascertained  by  actual  observation. 

Its  effect,  if  not  allowed  for,  is  always  to 
place  the  machine's  head  towards  the  direction 
of  the  old  course,  as  shown  in  sketch  (p.  17). 

The  variation,  dip,  horizontal  and  vertical 
force  are  all  given  in  Admiralty  publications. 

Reference  to  these  will  show  that  in  the 
south  of  England  the  dip  is  approximately  67°. 

It  will  also  be  noticed  that  as  the  latitude 
gets  higher  the  dip  increases,  and  therefore 
the  vertical  force  in  big  latitudes  is  greater 
than  the  horizontal  force.  Hence  it  is  neces- 
sary that  the  compass  should  be  kept  the 
greatest  distance  possible  from  vertical  or 
nearly  vertical  iron,  especially  the  ends,  in 
these  latitudes. 

The  effect  of  a  magnet '  end  on  '  to  a  single 
pole  of  a  compass  is  much  greater  than  that 
of  a  magnet  '  broadside  on/ 

The  proof  is  here  given  for  anyone  who 
may  be  interested  in  it. 

Proof. — AB  is  a  magnet  and  N  is  an 
isolated  north  pole  of  strengths  M  and  m 
respectively. 

18 


MAGNETISM 

First  consider  the  '  end-on  '  position,  where 
d  is  the  distance  from  the  centre  of  the  magnet 
to  the  isolated  pole,  and  L  is  the  length  of  the 
magnet. 

Then  the  force  acting  on  N  due  to  the  south 
pole  of  the  magnet  is  : 

Mm 


A     * 


FIG.  10. 


And  the  force  acting  on  N  due  to  the  north 
pole  of  the  magnet  is  : 
M.m 


As  these  two  forces  act  always  along  the 
same  straight  line  their  total  is  : 

Mm  Mm 


Mmd*  +  MmdL  +  Mm—  -  Mmd*  +  Mmdl  -  Mm—' 
4  4 


Mmdl. 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 


Now  in  the  '  broadside  on  '  position  : 
Force  due  to  south  pole  of  magnet  on  N  : 


( 


FIG.  ii. 


And    force   due    to  north  pole   of    magnet 

on  N  : 

Mw 


And  their  resultant  is  evidently  equal  to  the 
line  CN,  as  in  sketch  following,  and  which 
equals  2  Sine«. 

20 


MAGNETISM 

Hence  the  total  force  acting  on  N  due  to 
the  magnet  broadside  on  is  : 


x  2  Sine  a 


•  N 


FIG.  12. 

That  is  : 

Mm  L  MwL 


4  4 

Analysing  these  results  we    find    that  '  end 
on': 

2  MwL 


Total  force  = 


(T 
d'~~ 


21 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 
And  for  '  broadside  on  '  : 

T  4.  i  f  MwL 

Total  force  = 


(L2V 
d         'A) 


d*  d* 

Mm 


If  d  is  large  compared  with  L  we  can 
neglect  L,  and  the  equations  become  : 

'  End  on'     ' 

<  Broadside  on '       (^)f  ^3 

Showing  that  force  '  end  on '  is  twice   that 
'  broadside  on/ 

In  conclusion  it  should  be  understood  that 
the  magnetic  effect  exerted  by  any  object 
cannot  be  screened  off  from  any  object  liable 
to  be  influenced  by  magnetism,  if  the  latter 
falls  within  the  magnetic  field  of  the  former. 


22 


CHAPTER  II 
THE  MAGNETIC  COMPASS 

THIS  is  an  instrument  constructed  to  give 
the  direction  of  the  magnetic  north,  and  by 
means  of  a  graduated  card  fixed  to  it  to  give 
any  other  direction  with  relation  to  it. 

A  freely  suspended  magnetic  needle  would 
of  course  point  to  the  magnetic  north,  and  if  a 
card  were  attached  to  it  it  might  seem  at  first 
sight  that  this  would  fulfil  all  requirements  ; 
but  it  must  be  remembered  that  this  form  of 
suspension  would  be  affected  by  the  varying 
angle  of  dip,  and  would  therefore  only  be  actu- 
ally horizontal  when  on  the  magnetic  equator. 
In  any  other  place  it  would  have  a  varying 
angle  of  tilt  which  would  make  reading  awk- 
ward, whilst  in  high  latitudes  the  card  would 
come  up  against  the  glass  cover  of  the  compass 
bowl  and  prevent  the  card  from  working. 

Various  methods,  including  the  lowering  of 
the  centre  of  gravity,  have  been  devised  to 
overcome  this,  and  the  compass  card  as  now 

23 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

constructed  will  remain  horizontal  in  any  part 
of  the  world. 

As  the  card  remains  horizontal,  the  only 
force  we  need  consider  as  acting  on  the 
compass  card  is  the  horizontal  component  of 
the  earth's  magnetism. 

The  general  system  of  pivoting  a  compass 
card  is  as  follows. 

The  magnets  and  card  are  fixed  together, 
and  are  fitted  with  a  cap  in  their  centre  which 
is  inverted  and  fitted  with  a  ruby  or  other  hard 
stone  to  take  the  wear  and  also  to  reduce 
friction  to  as  little  as  possible.  This  is  then 
placed  on  to  a  metal  spike  which  is  given  an 
iridium  point,  the  latter  being  an  extremely 
hard  metal.  (Sapphire  or  ruby  points  will 
probably  be  used  in  future,  owing  to  the 
deterioration  in  quality  of  the  iridium  now 
being  mined.) 

In  all  the  later  patterns  of  aero  compasses 
the  above  arrangement  is  reversed,  the  pivot 
being  fixed  to  the  card. 

Owing  to  a  sticky  deposit  which  is  liable 
to  form  in  the  cap  this  would  at  first  sight 
seem  to  be  a  disadvantage,  but  the  fact  that 
it  gives  greater  steadiness,  coupled  with  the 
greater  angle  of  clearance  between  card  and 
covering  glass,  negatives  this  disadvantage. 

24 


THE  MAGNETIC  COMPASS 

The  card,  cap  and  pivot  are  enclosed  in  a 
non-magnetic  bowl  and  covered  with  a  glass 
cover. 

In  the  earlier  pattern  compasses  the  card 
used  to  work  in  air,  but  owing  to  the  great 
vibration  encountered  in  aeroplanes,  this  kind 
of  compass  was  found  to  be  totally  unsuitable, 
so  the  liquid  type  had  to  be  introduced  instead. 

Its  advantages  over  the  compass  card 
working  in  air  are  as  follows  : 

The  card  is  steadier,  it  takes  less  time  to 
settle  down  if  disturbed,  and  a  heavier  card 
may  be  used,  as  the  total  weight  resting  on 
the  pivot  may  be  made  to  any  amount  re- 
quired by  varying  the  size  of  the  float. 

The  size  of  the  bowl  is  such,  that  a  clearance 
of  about  one  quarter  of  its  diameter  is  allowed 
for  between  its  inner  edge  and  the  edge  of  the 
card,  otherwise  when  turning  rapidly  a  rotary 
motion  is  set  up  in  the  liquid  which  is  com- 
municated to  the  card.  This  makes  it  liable 
to  become  unsteady  or  to  lag  behind. 

The  Liquid  used  in  a  Compass. — This  is  a 
mixture  of  two  parts  of  distilled  water  to 
one  part  of  pure  alcohol,  the  object  of  the 
alcohol  being  to  prevent  freezing. 

This  mixture  is  quite  efficient  up  to  —2° 
25 


AIR  NAVIGATION  FOR   FLIGHT   OFFICERS 

Fahrenheit.  It  has  been  found  that  a 
slightly  higher  percentage  of  alcohol  gives 
better  results  in  very  low  temperatures,  and 
all  the  later  pattern  compasses  are  now  filled 
with  a  mixture  of  three  parts  of  distilled  water 
to  two  parts  of  alcohol. 

Distilled  water  only  must  be  used,  otherwise 
the  impurities  in  ordinary  water  would  clog 
up  the  cap  and  render  the  compass  sluggish. 

To  Remove  a  Bubble  from  a  Compass. — The 
fact  of  an  air  bubble  having  formed  in  a 
compass  can  always  be  seen.  It  should  never 
be  allowed  to  remain,  as  it  makes  steering 
difficult  and  also  tends  to  make  the  compass 
sluggish. 

The  following  procedure  should  be  carried 
out. 

The  bowl  should  be  removed  from  its 
outer  containing  case  and  laid  on  its  side  with 
the  filling  screw  uppermost.  Remove  the 
screw  plug  and  drop  in  distilled  water  with  a 
pipette  or  clean  fountain-pen  filler. 

Rock  the  bowl  gently  from  side  to  side 
to  make  sure  the  bubble  is  underneath  the 
filling  plug. 

As  soon  as  the  water  overflows  replace  the 
screw  plug  and  take  care  that  the  leather 

26 


THE  MAGNETIC  COMPASS 

washer  is  in  place.  If  on  examination  it  is 
found  that  all  the  air  is  not  yet  out  the  opera- 
tion must  be  repeated. 

The  bowl  should  be  as  cool  as  possible  so 
as  to  enable  the  maximum  amount  of  water 
to  be  introduced. 

Remarks    on    Placing    a     Compass. — The 

placing  of  a  compass  in  a  good  position  is  of 
great  importance,  as  should  a  bad  position 
be  chosen,  even  the  best  of  compasses  will 
be  unsatisfactory  in  their  behaviour.  The 
points  to  be  attended  to  are  as  follows  : 

(1)  It  should  be  placed  in  a  position  where 
the  pilot  has  a  clear  view  of  it,  and  if  possible 
in  the  centre  of  the  longitudinal  axis  of  the 
machine.       This  tends  to   make  the  errors 
more  symmetrical  and  therefore  more  easily 
adjusted. 

The  pilot  should  also  be  directly  behind 
the  compass,  to  avoid  errors  in  reading  due 
to  parallax. 

(2)  The  maximum  distance  possible  from 
magneto,  engine,  or  anything  magnetic  liable 
to  occasional  movement. 

(3)  If  possible,  all  metal  within  at  least 
2  feet  from  the  compass  should  be  made  of 
some  non-magnetic  material. 

27 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 

(4)  The  greatest  distance  possible  from  the 
ends  of  vertical  iron  rods,  struts,  etc. 

Essential  Features  in  an  Aeroplane  Compass. 

(1)  Steadiness  under  all  conditions. 

(2)  Good  expansion  arrangements. 

(3)  Satisfactory  system  of  lighting. 

(4)  Sufficient  allowance  between  card  and 
covering  glass  to  prevent  their  touching  each 
other  in  the  event  of  the  machine  climbing, 
planing,  or  banking. 

(5)  Good  marking  of  the  card  and  reduc- 
tion of  eye  strain  to  a  minimum. 

These  requirements  have  been  attained  as 
follows. 

(i)  Steadiness. — After  numerous  experi- 
ments it  was  found  that  the  most  efficient  way 
to  damp  the  existent  vibrations  was  to  place 
the  compass  bowl  in  a  bed  of  horse-hair.  This 
effectually  deadened  shocks. 

The  horse-hair  is  placed  in  light  outer 
containing  case,  and  the  bowl  rests  lightly 
on  it. 

This  method  was  found  generally  to  be 
much  more  preferable  to  the  old  gimballed 
type  of  suspension. 

This  applies  to  the  earlier  pattern  com- 
28 


THE  MAGNETIC  COMPASS 

passes ;  for  Pattern  255  and  later  types  the 
reader  should  refer  to  the  book  '  Magnetic 
Compass  in  Aircraft/  by  Captain  F.  Creagh 
Osborne,  R.N. 

(2)  Expansion  and  Contraction. — Due  pro- 
vision has  been  made  for  this  by  fitting  what 
is  known  as  an  '  expansion  chamber  '  in  the 
bowl. 

(3)  Lighting.— In    the   earlier   types   this 
was  arranged  for  by  a  small  dry  cell  battery 
and  electric  lamp  fitted  on  the  front  side  of 
the  bowl. 

In  the  later  types  this  method  is  of  second- 
ary importance,  as  the  card  markings  are 
treated  with  a  radium  compound  enabling  it 
to  be  easily  read  in  the  dark. 

(4)  Allowance      for      Heeling.— This      is 
arranged   for   by   the   method   of   pivoting, 
which  allows  of  a  heel  of  15°  in  the  earlier 
types ;   and  by  altering  the  pivoting  in  the 
later  types  this  has  been  increased  to  about 
30°.     These  are  the  angles  that  the  machine 
has  to  heel  over  to  before  the  card  touches 
the  covering  glass. 

(5)  Making  of  the  Card.— This   has  been 

29 


AIR  NAVIGATION  FOR   FLIGHT   OFFICERS 

done  in  what  is  known  as  the  '  New  Style/ 
i.e.  the  card  is  graduated  from  o°  to  360°, 
running  with  the  hands  of  a  watch. 

North  is  thus  represented  by  o°  or  360°, 
North-east  by  45°,  East  by  90°,  South-east  by 
135°,  South  by  180°,  South-west  by  225°,  West 
by  270°,  North-west  by  315°. 

North,  South,  East,  and  West  are  called  the 
'  Cardinal  Points/  North-east,  South-east, 
South-west,  and  North-west  are  called  the 
'  Quadrantal  Points/  Small  aeroplane  com- 
passes are  only  marked  every  5°  to  prevent 
overcrowding,  owing  to  the  small  size  of  the 
card.  The  number  is  given  opposite  every 
tenth  degree. 

Airship  compasses  are  graduated  to  every 
degree,  and  large  compasses  for  big  aeroplanes 
every  two  degrees. 

Prisms  and  Reflectors.— These  are  intro- 
duced to  do  away  with  eye  strain  as  much  as 
possible,  the  card  being  so  small. 

Broken  Pivots. — This  causes  the  card  to 
work  jerkily,  and  the  compass  should  be  taken 
apart  and  the  pivot  examined  to  see  whether 
it  is  bent  or  damaged.  If  it  cannot  be  repaired 
the  compass  should  be  returned  to  store,  and 
a  new  one  drawn  in  lieu. 

30 


THE  MAGNETIC  COMPASS 

Magnet  Block. — These  are  supplied  for 
holding  the  adjusting  magnets.  They  should 
be  placed  so  that  their  centre  is  directly  under 
the  centre  of  the  compass,  and  care  should  be 
taken  that  the  magnet  holes,  of  which  there 
are  two  sets  at  right  angles  to  each  other, 
should  be  set  so  that  they  are  in  line  respec- 
tively with  the  longitudinal  and  transverse 
axes  of  the  machine.  These  blocks  will  not 
be  met  with  in  Pattern  255  and  later  types. 

Effect  of  Banking. — The  effect  after  a 
heavy  bank  is  to  make  the  compass  unsteady 
for  a  short  time.  It  has  been  found  by 
experiment  that  if  on  a  fast  machine  steering 
anywhere  within  20°  of  the  north  point,  a 
quick  alteration  of  course  will  cause  the  north 
pole  of  the  compass  to  follow  the  machine's 
head  round.  On  steadying  the  machine,  the 
north  pole  of  the  compass  swings  back  to  its 
correct  position. 

For  a  description  of  the  various  types  of 
compasses  used  in  aircraft,  reference  should  be 
made  to  the  pamphlet  entitled  '  Compasses 
for  Use  in  Aircraft/  by  Captain  F.  Creagh 
Osborne,  R.N. 


CHAPTER  III 

THE  ANALYSIS  AND  ADJUSTMENT 
OF  DEVIATION 

THE  effect  of  the  magnetic  qualities  in  hard 
and  soft  iron  is  to  deflect  the  compass  needle 
from  the  magnetic  meridian.  This  deflection, 
or  local  attraction,  as  it  is  otherwise  called, 
is  known  as  '  deviation/ 

For  purposes  of  analysis  and  adjustment, 
this  deviation  can  be  split  up  into  five  '  co- 
efficients/ as  they  are  called,  viz.  A,  B,  C,  D, 
and  E. 

These  coefficients,  with  the  exception  of 
A,  may  be  assumed  to  be  acting  immediately 
over  or  under  the  centre  of  the  compass, 
longitudinally,  transversely,  or  diagonally. 
Coefficients  A,  D,  and  E  are  caused  by  soft 
iron,  and  B  and  C  by  hard  iron. 

Coefficient  A. — Is  due  to  iron  being  un- 
symmetrically  distributed  around  the  com- 
pass, or  is  due  to  the  latter  being  out  of  the 
middle  longitudinal  line  of  the  machine. 

32 


COEFFICIENTS 

It  is  extremely  rare  in  a  well -placed  compass. 

An  '  apparent '  A  may  be  caused  by  an 
error  in  the  magnetic  bearing  of  an  object 
which  is  being  used  for  swinging.  In  practice 
it  will  be  found  that  nearly  every  aeroplane 
compass  has  an  'A/ 

It  cannot  be  corrected  but  can  only  be 
allowed  for.  It  is  called  a  '  constant '  devia- 
tion, because  it  is  the  same  in  amount  and  sign 
for  all  directions  of  the  machine's  head. 

It  is  found  by  taking  the  mean  of  the  devia- 
tions on  a  number  of  equidistant  points, 
calling  all  easterly  deviations  +  and  all 
westerly  deviations  — .  In  practice,  it  is  usual 
to  take  the  deviations  on  the  cardinal  and 
quadrantal  points. 

Coefficient  B. — Is  caused  by  the  hori- 
zontal component  of  the  permanent  magnet- 
ism of  the  machine  acting  longitudinally. 

It  is  called  +  if  the  north  end  of  the  needle 
is  drawn  towards  the  nose  of  the  machine, 
and  -  if  drawn  towards  the  tail. 

It  is  maximum  on  east  and  west,  diminish- 
ing to  zero  on  north  and  south. 

It  is  found  by  taking  the  mean  of  the 
deviations  on  east  and  west,  changing  the  sign 
on  west. 

33  D 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

It  is  corrected  by  horizontal  magnets, 
placed  red  end  to  the  front  for  a  +  B,  and  red 
end  to  the  rear  if  B  is  -.  It  causes  a  semi- 
circular deviation,  so  called  because  its  sign 
changes  once  only. 


315 

Wly.Devn, 


225 

Wly.  Devn 


ISO 


315° 


FIG.  13. — Diagram  for  +  B. 


Curve  for  _j_  B. 


It  varies  inversely  as  the  horizontal  force, 

e.g.  Hor.  Force  I.  B  =  10°  +  say 
Then  with  Hor.  Force  2.  B  =  J  x  10°  - 

=  5°+- 
34 


COEFFICIENTS 


Diagram  for  +  B.     (Fig.  13.) 

The  shaded  rod  denotes  the  permanent 
magnetism  acting  longitudinally.  The  pecked 
arrow  denotes  the  direction  in  which  the  needle 
is  deflected. 


Ely.  Devn 


90 

Wly.  Devn. 
Max. 


225 
Ely.  Devn 


FIG.  14. — Diagram  for  —  B. 


135 

Wly.  Devn. 


180" 


270 


1 


360° 

Curve  for  —  B. 


Diagram  for  —  B.     (Fig.  14.) 

The  above  remarks  apply  here  as  well. 

Coefficient  C. — Is  caused  by  the  hori- 
zontal component  of  the  machine's  permanent 
magnetism  acting  transversely. 

35 


AIR   NAVIGATION   FOR    FLIGHT   OFFICERS 

It  is  called  +  if  the  north  end  of  the  needle 
is  drawn  to  the  right-hand  side  of  the  machine 
and  -  if  drawn  to  the  left-hand  side. 

It  is  maximum  on  north  and  south,  dimin- 
ishing to  zero  on  east  and  west. 

It  is  found  by  taking  the  mean  of  the 
deviations  on  north  and  south,  changing  the 
sign  on  that  of  south.  It  is  corrected  by 
horizontal  magnets  placed  transversely,  red 
end  to  the  right  if  C  is  -f  and  red  end  to  the 
left  if  C  is  -. 

It  is  called  '  semicircular  '  for  the  same 
reason  as  B,  and  like  B  changes  inversely  as 
the  horizontal  force. 

Diagram  for  +  C.     (Fig.  15.) 

The  shaded  rod  denotes  the  permanent 
magnetism  acting  tiansversely,  and  the 
pecked  arrow  the  direction  the  needle  is 
deflected  to. 

Diagram  for  --  C.     (Fig.  16.) 
See  remarks  above. 

Coefficient  D. — Is  the  effect  of  induction 
in  horizontal  soft  iron  acting  longitudinally 
or  transversely. 

If  the  effect  of  the  transverse  iron  is  greater 
than  that  of  the  longitudinal  it  causes  a  +  D, 

36 


COEFFICIENTS 

and  if  the  effect  of  the  longitudinal  is  greater 
it  causes  a  -  D. 

D  is  maximum  on  the  quadrantal  points 
(i.e.  45°,  135°,  225°,  315°),  diminishing  to  zero 
on  the  cardinal  points. 


315 
Ely.Devn 


225 

Wly.  Oevn 


FIG.  15. — Diagram  for  -f-  C. 


Curve  for  -f-  C. 


It  is  found  by  taking  the  mean  of  the 
deviations  on  the  semicardinal  points,  chang- 
ing the  signs  on  south-east  and  north-west. 
That  is  on  135°  and  315°. 

It  is  corrected  by  soft  iron  spheres  placed 

37 


AIR  NAVIGATION   FOR   FLIGHT  OFFICERS 

transversely  if  D  is  -f  and  longitudinally  if 
Dis  -. 

The  size  of  the  spheres  and  the  distance  of 


315 

Wly  Devn 


225 

Ely.  Devn 


Ely.  Devn 
ISO         Max 


FIG.  1 6. — Diagram  for  —  C. 


Curve  for  —  C. 


their  centre  from  the  centre  of  the  compass  is 
given  in  tables  to  be  found  in  the  '  Admiralty 
Compass  Manual/ 

D  is  called  '  quadrantal '  because  it 
changes  its  sign  in  each  quadrant.  It  does  not 
change  on  change  of  position  because  the 
force  acting  on  the  iron  is  the  same  as  that 

38 


COEFFICIENTS 

acting  on  the  compass  needle,  and  the  two  are 
therefore  always  in  proportion,  as  shown 
below. 


(a)  Hor.  Force  1 

i 
l  t          1 


Proportion  -i-  =  l 


(b)  Hor.  Force  2 

2 
2  '  2 


Proportion 


FIG.  17. 


225 

Ely.  Devn 
Max. 


180 
FIG.  1 8. — Diagram  for  +  D. 


39 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 


Diagram  for  +  D.     (Fig.  18.) 

The  white  rods  denote  that  the  iron  is 
non-magnetic. 


315 
Wly.Devn 

Max. 


225 

Wly.  Devn. 
Max. 


FIG.  19. — Diagram  for  —  D. 


Curve  for 


The  other  coefficient  is  called  E  and  is 
caused  by  iron  running  obliquely,  but  it  is 
not  proposed  to  go  into  it  here. 

The  Effect  of  Vertical  Iron. — Vertical  iron 
causes  what  is  known  as '  Heeling  Error/  which 
comes  into  action  on  the  machine  banking. 

40 


ANALYSIS  OF  DEVIATION 

This  is  maximum  on  north  and  south,  and 
zero  on  east  and  west. 

Analysis  of  a  Table  of  Deviations. — By 
this  is  meant  the  splitting  up  of  a  table  of 
deviations  into  the  various  coefficients,  which 
is  done  by  following  the  rules  given  in  the 
explanation  of  the  various  coefficients. 

A  worked  example  is  here  given. 

Analyse  the  following  table  of  deviations  : 


Machine's 
Head. 

0° 

o 


45 


° 


QO 

-35 
Coefficient  A. 


135° 


Deviation. 

Machine's 
Head. 

Deviation. 

3-W- 

180° 

5-E.  + 

4-E.+ 

225° 

3.W.- 

7-E.+ 

270° 

8.W. 

lo.E.  + 

315° 

6.W. 

4  3 
7  3 

10  8 

5  6 


20 


41 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

Coefficient  B.  Coefficient  C. 

+  7  -3 

+  8    Original  sign  —5      Original  sign 

2  |  +15      changed.  2  |  —8  changed. 

+  7°  30'.  -4°  oo' 

Coefficient  D. 


+ 

— 

-13 

4 

10      Sign 

+  10 

6      Sign 

changed. 

4-1--  3 

changed. 

3 

-o-  45' 

10 

13 

Coefficient  E. 

5  3  4 1 +3 


j*_  J7  +o°45' 

13  10 

Correctors  would  then  be  placed  as  follows : 

B  would  be  corrected  by  magnets  placed 
longitudinally,  red  end  in  front. 

C  would  be  corrected  by  transverse  mag- 
nets, red  end  to  left. 

D  would  be  corrected  by  spheres  placed 
longitudinally,  the  size,  etc.,  being  taken  from 
the  tables. 

But  in  this  case,  D  being  so  small,  it  would 
be  best  to  ignore  it  altogether. 

42 


CHAPTER  IV 

THE  PRACTICAL  CORRECTION  OF  A 
COMPASS;  METHODS  OF  SWING- 
ING, ETC. 

BEFORE  going  into  the  practical  correction  of 
a  compass,  it  is  proposed  to  give  a  description 
of  the  various  methods  of  swinging. 

This  swinging  should  always  be  carefully 
carried  out,  as  a  well-placed  compass  whose 
behaviour  is  good,  and  whose  errors  are  known 
and  can  be  trusted,  is  a  great  relief  to  a  pilot 
making  a  flight,  when  objects  below  are  hidden 
by  cloud,  fog,  etc. 

There  are  five  methods  of  swinging,  as 
follows : 

(a)  By  Sun  or  Star. 

(b)  By  '  Reciprocal  Bearings/ 

(c)  By  Distant  Object. 

(d)  By  two  objects  in  line  or  '  in  transit/ 
as  it  is  usually  called. 

(e)  By  using  a  marked-out  flying  ground. 

43 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

In  connection  with  this,  the  last-mentioned 
one  is  the  only  one  possible  for  the  later  pat- 
tern compasses,  owing  to  their  construction. 

(a)  By  Sun  or  Star. — The  requirements 
for  these  are  a  watch  whose  error  on  Green- 
wich mean  time  is  known,  a  shade  for  use  if 
the  sun  is  observed,  notebook  and  tables  for 
giving  the  true  bearing  of  the  body  at  various 
intervals  of  time. 

The  body  observed  should  be  fairly  low  in 
altitude. 

Place  the  machine's  head  in  the  required 
direction  by  compass,  and  observe  bearing  of 
the  body,  noting  the  time  of  doing  so  by  the 
watch.  Transfer  this  watch  time  into  appar- 
ent time  at  place  (this  will  be  explained  in 
the  chapter  on  Astronomy),  and  look  up  the 
body's  declination. 

With  these  data,  and  knowing  your 
latitude,  the  true  bearing  can  be  looked  out 
from  the  tables. 

Apply  the  variation  to  this  to  get  the 
magnetic  bearing.  The  difference  between 
the  true  and  magnetic  bearings  will  be  the 
deviation  for  that  particular  direction  in 
which  the  machine  is  heading. 

This  operation  should  be  repeated  for  all 
44 


METHODS  OF  SWINGING 

required  directions.  In  practice  it  is  custom- 
ary to  work  out  a  table  of  times  and  magnetic 
bearings  in  advance,  as  it  much  facilitates 
operations. 

This  method  is  always  used  at  sea  when  out 
of  sight  of  land,  but  is  not  of  much  practical 
value  on  a  flying  ground. 

(b)  By  *  Reciprocal  Bearings/ — For  this 
purpose  a  compass  known  as  the  '  Landing 
Compass/  or  '  Shore  Compass/  is  set  up  in 
some  place  on  the  flying  ground  where  it  will 
be  free  from  all  local  attraction  in  the  shape 
of  sheds,  adjacent  machines,  etc. 

This  ensures  it  being  free  from  deviation. 

The  machine  to  be  swung  is  wheeled  out 
and  also  placed  in  a  position  similar  to  the 
other  compass,  and  heading  in  any  required 
direction. 

Simultaneous  bearings  are  taken  of  the 
shore  compass  by  the  machine's  compass, 
and  of  the  machine's  compass  by  the  shore 
compass. 

Either  of  these  bearings  should  now  be 
reversed  and  the  difference  between  this 
reversed  bearing  and  the  other  one  will  be 
the  deviation  for  that  particular  direction  of 
the  machine's  head. 

45 


AIR  NAVIGATION   FOR   FLIGHT   OFFICERS 

This  operation  can  be  repeated  on  any 
other  direction  of  the  machine's  head. 

Examples : 

(1)  Bearing  of  shore  compass      .        .     227° 
Bearing  of  machine's  compass      .       40° 

Reversed  bearing  by  shore  compass      47° 
Bearing  by  machine's  compass     .       40° 

Deviation  7°  E. 

(2)  Bearing  by  shore  compass     .        .       62° 
Bearing  by  machine's  compass     .     247° 

Reversed  bearing  by  shore  compass    242° 
Bearing  by  machine's  compass     .     247° 

Deviation      .         5°  W. 

The  shore  compass  should  be  set  up  some 
little  distance  from  the  machine,  not  less  than 
fifteen  or  twenty  yards. 

(c)  By  Distant  Object.—  This  is  a  very  easy 
method,  as  it  entails  the  use  of  no  instru- 
ments, and  only  one  observer  is  needed. 

The  magnetic  bearing  of  some  distant 
object  having  been  found  beforehand,  from  a 
particular  spot,  the  machine  is  wheeled  out 
and  placed  so  .that  the  compass  is  over  this 
spot,  and  heading  in  the  required  direction. 
All  that  has  to  be  done  now  is  to  take  the 

46 


•  METHODS  OF  SWINGING 

bearing  of  the  distant  object  by  the  compass, 
and  repeat  it  on  any  other  direction. 

The  difference  between  the  compass  bear- 
ing of  the  distant  object  and  the  magnetic 
bearing  already  found,  will  be  the  deviation  for 
that  particular  direction  of  the  machine's  head. 


Examples  : 

Machine's 
Head. 

Magnetic 
Bearing. 

Compass 
Bearing. 

Deviation. 

0° 

309° 

314° 

5°W. 

45°  309°  3H°  2°  W. 

90°  309°  306°  3°E. 

(d)  By  Two  Objects  in  Line. — This  is  the 
same  as  Case  (c).     But  in  place  of  one  object 
there  are  two  in  line,  the  magnetic  bearing  of 
one,  and  therefore  of  both,  being  known. 

This  case  is  valuable  for  checking  devia- 
tion, as  the  magnetic  bearing  can  be  obtained 
from  the  chart ;  and  when  flying,  as  soon  as  the 
objects  come  into  line,  the  bearing  can  betaken. 

This  will  show  at  once  whether  or  not  the 
deviation  has  altered. 

(e)  By  a  Marked-out  Flying  Ground. — This 
is  the  simplest  method  of  all,  requiring  no 
instruments  and  no  objects,  and  a  machine's 
compass  can  be  adjusted  at  any  hour  of  the 
day  or  night,  and  also  in  thick  weather  when 
all  distant  objects  and  marks  are  obscured. 

47 


AIR   NAVIGATION   FOR   FLIGHT  OFFICERS 

The  spot  having  been  chosen,  permanent 
lines  are  marked  out  running  north,  south, 
east,  and  west.  The  north-east,  south-east, 
south-west,  and  north-west  lines  may  also 
be  drawn  in  if  required.  Permanent  marks 
should  be  placed  at  the  ends  of  these  lines  and 
also  at  the  central  spot. 

All  that  has  to  be  done  is  to  place  the 
machine's  head  along  the  desired  line  and 
note  the  compass  reading. 

The  difference  between  this  and  the  lubber 
point  of  the  compass  will  be  the  deviation. 

An  explanation  of  the  methods  of  marking 
out  a  flying  ground  will  now  be  given. 

In  the  working  of  the  following  example, 
the  explanation  of  the  various  terms  used  will 
be  found  in  the  chapter  on  Astronomy. 

The  marking  out  of  a  flying  ground  can 
be  done  in  two  ways. 

(a)  By  means  of  the  shore  compass,  set  up  in 
any  convenient  spot  free  from  local  attraction. 

The  lines  can  be  got  straight  away  by 
direct  observation,  and  marked  in. 

(b)  An    alternative    method,    which    in- 
volves a  little  more  trouble,  but  once  done 
holds  good  as  long  as  the  first  case.     It  con- 
sists of  finding  the  magnetic  bearing  of  one 
or  more  conspicuous  objects  visible  from  the 

48 


MARKING  OUT  A  FLYING  GROUND 

swinging  ground,  and  from  this  bearing  to 
get  the  magnetic  directions  required.  The 
magnetic  bearing  of  one  of  the  objects  is 
obtained  by  simply  taking  a  horizontal  angle 
between  the  sun's  limb  and  the  object 
required.  The  sun's  bearing  can  now  be 
worked  out  and  this  angle  applied  to  it. 

The  result  will  be  the  true  bearing  of  the 
object,  so,  to  get  the  magnetic  bearing,  the 
variation  must  be  applied.  It  is  just  as  well 
to  have  the  bearings  of  two  or  three  objects 
in  case  one  is  done  away  with,  so  if  angles 
between  the  first  object  and  one  or  two  others 
be  taken,  they  can  be  applied  to  the  bearing  of 
the  first. 

An  example  of  this  follows. 

On  April  14,  1916,  at  a  certain  flying 
ground  in  Latitude  51°  North,  Longitude 
3°  West,  it  was  desired  to  lay  out  magnetic 
lines  for  compass  adjustment. 

The  following  observations  were  made. 

Rough  time  about  5.45  A.M. 

The  watch,  which  was  slow  on  Greenwich 
mean  time  o  hrs.  2  min.  15  sec.,  showed 
5  hrs.  55  min.  33  sec.  At  the  same  time 
the  observed  horizontal  angle  between  an 
object  A  to  the  right  of  the  sun  and  the 
sun's  near  limb,  was  97°  50'. 

49  E 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

The  following  angles  were  also  observed. 
Right  of  A.  Left  of  A. 

Object  B    64°  40'     Object  C    37°  50' 

Variation  16°  W.  Required  the  magnetic 
bearings  of  A,  B,  and  C. 

N.B. — In  this  example  the  working  is  not 
rigorously  exact,  but  is  near  enough  for 
practical  purposes. 


Time  by  watch 
SlowonG.M.T. 


h.    m.    s. 

5  55  33 
2  15 


G.M.T.  5  57  48 
Long,  in  time  .  12  oo 

Mean  time  at  place .       5  45  48 
Equation     of    time 

from        Nautical 

Almanac     .          .  -|~  20 


Apparent      time      at 

place  .          .       5  46  08 


Right  of  A. 
Magnetic  bearing  of 

A      . 
Angle  to  B    .          .        64°  40' 

Magnetic  bearing  of 

B      .  260°  03' 


195°  23' 


True  bearing  sun's 
limb  from  table 

Sun's  semidiame- 
ter  from  Nau- 
tical Almanac  . 


81°    if 


+      16' 

True  bearing  sun's 

centre      .          .       81°     33' 
Angle  to  A  right 

of  sun       .          .       97°  50' 


True  bearing  of  A     179°  23' 
Variation   . 


o 

i6°oo/W. 


Magnetic  bearing 

of  A  .       195°  23' 

Left  of  A. 
Magnetic  bearing  of 

A  .          .          .       195°  23' 
Angle  to  C  .         37°  5o' 


Magnetic  bearing  of 


157°  33 


MARKING  OUT  A  FLYING  GROUND 

To  get  the  sun's  true  bearing  from  the 
tables,  we  require  to  know  three  things : 
the  latitude,  the  sun's  declination,  and  the 
sun's  hour  angle. 

The  latitude  we  know  already,  the  hour 
angle  will  be  the  apparent  time,  since  we  keep 
our  time  from  the  sun,  and  the  declination 
can  be  taken  out  of  the  Nautical  Almanac  for 
that  day  at  sight.  The  declination  is  given 
for  noon  each  day,  but  as  its  total  change 
for  twenty-four  hours  is  comparatively  small, 
this  can  be  neglected,  as  it  is  near  enough 
for  compass  work. 

The  bearing  given  in  the  tables  is  that  of 
the  sun's  centre,  so  to  get  the  bearing  of 
the  sun's  limb,  the  semidiameter  must  be 
applied.  Whether  to  add  or  subtract  it  can 
easily  be  ascertained  from  a  figure ;  the  one 
on  p.  52  is  the  one  for  the  example  given. 

NOA  is  the  angle  given  in  the  tables,  and 
NOB  is  the  angle  required.  As  A  is  to  the 
right  of  the  sun,  the  angle  AOB  is  additive 
to  the  angle  NOA. 

It  should  be  remembered  that  the  observer 
is  at  O  facing  the  sun. 

Therefore,  in  this  case,  the  semidiameter 
must  be  added  to  the  bearing  from  the 
tables. 


AIR  NAVIGATION    FOR   FLIGHT   OFFICERS 

The  semidiameter,  in  the  case  of  compass 
work,  may  be  taken  as  a  constant  of  16'. 
Having  found  the  magnetic  bearing  of  A 


270° 


I80C 


'To  A 

FIG.    20. 


to  be  195°  23',  it  follows  that  the  magnetic 
north  must  lie  either  195°  23'  to  the  left  of  A, 
or  360°  oo'  —  195°  23",  i.e.  164°  37'  to  the  right 
of  A  as  shown  in  the  following  sketch. 

To    lay    out    the    ground,  the    following 
procedure  should  be  adopted. 

52 


;    :    MARKING  OUT  A  FLYING  GROUND 

Set  the  vernier  of  the  verge  plate  to  zero, 
and  having  placed  the  landing  compass  over 
the  central  position  on  the  swinging  ground, 
turn  the  whole  bowl  of  the  compass  round 
until  the  object  A  is  seen  in  the  prism  slit 
in  line  with  the  sight  wire. 


Now  set  the  vernier  either  195°  23'  to  the 
left  of  A  or  164°  37'  to  the  right  of  A.  The 
sight  wire  will  now  be  pointing  direct  to 
the  magnetic  north. 

Get  someone  to  walk  slowly  across  your 
line  of  sight  with  a  peg,  and  stop  him  when  he 
comes  in  line  with  the  sight  wire.  Drive  this 

53 


AIR   NAVIGATION   FOR   FLIGHT  OFFICERS 

peg  into  the  ground,  this  will  represent  the 
north  point  from  the  position  of  the  compass. 
Taking  this  north  point  as  the  starting  point, 
the  remaining  points  of  the  compass  can  now 
be  pegged  out  in  turn  and  the  lines  painted 
in  if  required,  the  pegs  being  left  standing  or 
replaced  by  small  base  plates  flush  with  the 
ground. 

The  advantage  of  having  two  or  more 
marks  whose  bearing  is  known,  lies  in  the  fact 
that  some  of  them  may  be  destroyed  in  course 
of  time,  in  which  case  bearings  to  new  marks 
would  have  to  be  found. 

If  it  is  desired  to  lay  out  more  than  one 
swinging  ground,  the  bearing  may  be  found 
from  one  position  and  calculated  for  the 
others  from  the  chart  or  map  on  the  largest 
scale  possible. 

When  taking  the  horizontal  angle  between 
the  sun  and  an  object,  always  have  the  sun  as 
low  in  altitude  as  possible. 

The  Practical  Correction  of  a  Compass.— 
By  this  is  meant  the  actual  placing  of  the 
adjusting  magnets  to  neutralise  the  effect  of 
the  iron  surrounding  the  compass. 

If  the  machine  is  a  new  one,  it  should  be 
swung  for  its  natural  deviations  on  the  eight 

54 


PRACTICAL  CORRECTION 

principal  points  of  the  compass  by  one  of  the 
methods  already  described. 

By  '  natural '  deviations  is  meant  the 
deviations  of  the  compass  before  any  of  the 
correctors  are  applied. 

These  deviations  having  been  ascertained, 
the  coefficients  can  be  worked  out  and 
the  various  correctors  placed  in  position 
roughly. 

Coefficient  D  should  always  be  corrected 
first,  if  intended  to  correct  it,  its  amount  and 
requisite  size  of  spheres  being  obtained  from 
the  published  tables. 

This  can  be  done  with  the  machine  head- 
ing in  any  direction,  and  when  once  done, 
holds  good  for  any  place  the  machine  may 
be  in. 

Now  place  the  nose  of  the  machine  north 
or  south,  or  east  or  west ;  and  correct  the 
coefficients  C  and  B  by  adjusting  the  trans- 
verse and  longitudinal  magnets  respectively 
as  necessary. 

This  is  done,  in  the  case  of  taking  a  distant 
object,  by  so  adjusting  the  magnets  as  to 
make  the  compass  bearing  of  the  object  agree 
as  nearly  as  possible  with  the  magnetic 
bearing  previously  found. 

If,  however,  the  machine  is  being  swung  on 
55 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 

a  marked-out  flying  ground,  it  is  only  neces- 
sary to  place  the  machine  heading  along  the 
lines  on  the  ground,  and  make  the  compass 
point  accurately  by  altering  the  position  of 
the  magnets  as  requisite. 

Having  done  this,  all  that  remains  to  be 
done  is  to  again  swing  the  machine  and 
tabulate  the  remaining  deviations,  which 
will  be  the  deviations  to  be  used  when  flying. 

If  the  compass  has  been  swung  before,  it 
will  only  be  necessary  to  readjust  the  magnets, 
if  required,  by  placing  the  nose  of  the  machine 
in  the  requisite  directions  and  making  the 
compass  bearing  agree  as  nearly  as  possible 
with  the  magnetic  bearing. 

The  machine  should  then  be  swung  as 
before,  to  get  the  remaining  deviations. 


CHAPTER  V 

CORRECTING  COURSES.  NAMING 
DEVIATION  RULES  FOR  GETTING 
THE  CORRECT  ANGLE  FROM  THE 
BEARING  TABLES.  FINAL  NOTES 

On  the  Correction  of  Courses. — A  knowledge 
of  how  to  apply  the  variation  and  deviation 
to  different  courses  in  a  correct  manner  is  of 
great  importance,  as  by  doing  so  wrongly  in  the 
case  of  the  variation  only,  the  pilot  may  find 
himself  flying  on  a  course  about  30°  from 
his  right  direction  if  the  variation  is  15°. 

If,  however,  the  following  rules  be  learnt 
and  attended  to,  he  need  never  get  into  that 
position. 

Rules. — The  following  rules  apply  to  varia- 
tion and  deviation  alike. 

(a)  Given  compass  course  or  magnetic 
course  to  find  true  course. 

Add  easterly.     Subtract  westerly. 

57 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

Examples : 

Compass  course  228°.  Variation  17°  E. 
Deviation  4°  W.  Find  true  course. 

Compass  Course    .          .          .       228°  j 
Variation      .          .          .          .      +17° 

245° 
Deviation     .         .         .         .       —  4° 

True  Course  .         .         .       241° 

Magnetic  course  163°.  Variation  14°  W. 
Find  true  course. 

Magnetic  Course   .         .         .       163° 
Variation      .          .         .         .     — 14° 

True  Course.          .          .          .       149° 

(b)  Given  true  course  to  find  magnetic  or 
compass  course. 

Add  westerly.     Subtract  easterly. 

True  course  117°.  Variation  14°  W. 
Deviation  6°  E.  Find  magnetic  and  compass 
courses. 

True  Course.         .         .         .       117° 
Variation      .         .         .         .     -fi4°W. 

Magnetic  Course   .         .         .        131° 

58 


CORRECTING  COURSES 

True  Course.         .         .         .       117° 
Variation      .          .          .          .      +14° 


131° 


Deviation      ...  — 6C 


Compass  Course    .         .         .       125° 

Should  the  result,  after  correction,  be 
found  to  exceed  360°,  the  latter  amount  must 
be  subtracted  from  the  total. 

Example : 

Compass  course  355°.     Variation  15°  E 
Deviation  5°  E.     Find  true  course. 

Compass  Course    .          .  355° 

Variation      .          .          .          .      -{-15° 


370° 


Deviation     .          .          .          •        +5° 

True  Course.         .         .         .       375° 

-360° 


True  Course.  15 


Notes  on  How  to  Name  Deviation. — Naming 
deviation  is,  to  a  novice,  at  first  a  little  diffi- 
cult, but  if  Figs.  22  and  23  be  studied,  it 

59 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 


270 


180° 

Deviation  5°E. 


FIG.  22. — Showing  easterly  "deviation. 


270C 


FIG.  23. — Showing  westerly  deviation. 


NAMING  DEVIATION 

will  at  once  become  apparent,  and  after  a 
little  practice  it  can  be  done  without  any 
writing  down. 

In  either  figure  the  circle  is  supposed  to 
represent  the  compass  card,  and  AB  the  line 
joining  the  distant  object  to  the  centre  of  the 
card. 

This  line  AB  should  be  considered  as 
absolutely  fixed,  and  in  Fig.  22  suppose  it  to 
run  in  the  direction  of,  say,  235°  magnetic. 

The  compass  card  is  free  to  revolve  about 
its  centre,  at  B,  and  in  this  case  the  degree 
230°  is  found  by  observation  to  be  lying  under 
the  line  AB. 

Therefore  the  235th  degree  must  have 
moved  in  the  direction  shown  by  the  black 
arrow. 

Now,  if  one  part  of  the  card  moves,  the 
whole  must  move  in  the  same  direction  ;  hence, 
if  we  follow  the  card  round  to  the  north 
point,  the  latter  must  clearly  move  in  the 
direction  shown  by  the  pecked  arrow. 

As  the  direction  of  the  pecked  arrow  is 
eastward,  the  deviation  must  be  easterly. 

In  Fig.  23,  suppose  the  line  AB  to  run,  say, 
132°.  From  observation  we  find  the  degree 
138°  to  be  under  this  line.  The  card's  motion 
must  have  been  in  the  direction  of  the  black 

61 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

arrow,  and  following  its  motion  round,  we 
see  the  north  point  of  the  card  must  move  in 
the  direction  of  the  pecked  arrow.  Hence  the 
deviation  must  be  westerly.  Therefore,  for 
any  card  graduated  according  to  the  new 
style,  i.e.  from  o°  to  360°,  the  rule  is  as 
follows : 

//  the  compass  bearing  is  less  than  the 
magnetic  bearing,  the  deviation  is  easterly ; 
if  greater  than  the  magnetic  bearing,  the  de- 
viation is  westerly. 

Notes  on  the  True  Bearings  taken  from  the 
Tables. — With  reference  to  the  bearings  taken 
from  the  tables,  it  must  be  remembered  that 
these  tables  were  made  out  for  the  old  pattern 
graduation  of  the  card,  and  therefore  require 
some  manipulation  before  the  bearing  by  the 
new  style  of  card  can  be  written  down. 

The  following  rules  should  be  well  learnt : 

(a)  In  north  latitude. 

If  the  time  is  A.M.,  the  bearing  may  be  taken 
straight  out  of  the  tables  and  written  down. 

If  the  time  is  P.M.,  the  bearing  given  in  the 
tables  must  be  subtracted  from  360°  and  the 
result  written  down  as  the  bearing  to  be  used. 

(b)  In  south  latitude. 

If  the  time  is  A.M.,  subtract  the  bearing 
62 


NOTES J3N  TRUE  BEARINGS 

given  in  the  tables  from  180°,  and  use  the 
result. 

If  the  time  is  P.M.,  add  180°  to  the  bearing 
given  in  the  tables,  and  use  the  result. 

N.B.— The  bearings  in  the  tables  are 
always  given  from  the  pole  of  the  observer's 
hemisphere. 


FIG.  24. 

These  rules  will  now  be  illustrated  graphic- 
ally by  figures. 

Fig.  24.    NORTH  LATITUDE.    A.M.  Time. 

NBC  is  the  angle  given  in  the  tables  and 
is  the  one  required. 

63 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

N 


NOTES  ON  TRUE  BEARINGS 

Fig.  25.    NORTH  LATITUDE.    P.M.  Time. 

NBC  is  the  angle  given  in  the  tables,  but 
the  angle  NESC  is  the  one  required,  i.e. 
360°  -  NBC. 

Fig.  26.    SOUTH  LATITUDE.    A.M.  Time. 


SBC  is  the  angle  given  in  the  tables,  and 
NBC  the  angle  required,  so  180°  -  SBC  = 
NBC. 

Fig.  27.    SOUTH  LATITUDE.    P.M.  Time. 

SBC  is  the  angle  given  in  the  tables,  and 
65  F 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

NESC  the  angle  required,   so  that   180°  -f 
SBC  =  NESC. 

To  Test  a  Compass. — This  should  be  done 
now  and  again  to  see  if  the  cap  and  pivot  are 
in  good  working  order,  as  they  are  liable  to 
damage  from  shocks  in  landing,  etc. 

This  can  be  done  in  two  different  ways. 

(1)  By  comparing  it  with  another  compass 
which  is  known  to  be  accurate. 

The  two  compasses  should  be  placed  as 
near  to  one  another  as  possible  without  inter- 
fering with  each  other's  field.  Bearings  of  a 
distant  object  as  far  away  as  possible  should 
be  taken  by  both  compasses  on  various  direc- 
tions of  the  aeroplane's  head. 

The  bearings  taken  by  the  machine's 
compass  corrected  for  the  known  deviation 
should  be  practically  the  same  as  the  bearing 
shown  by  the  other  compass. 

Should  they  differ  by  any  moderate 
amount,  the  cap  and  pivot  should  be  examined. 

(2)  By  deflecting  the  card  about  a  point 
from  its  normal  position  of  rest,  and  noting 
if  it  returns  to  its  old  position.     If  not,  it  is 
probable  that  something  is  wrong. 


66 


CHAPTER  VI 
METEOROLOGY 

IN  this  chapter  a  few  notes  will  be  given  of 
the  relation  of  wind  and  weather,  and  from  a 
study  of  these  it  is  hoped  that  the  pilot  may 
be  able  to  deduce,  from  his  own  observations, 
the  type  of  weather  he  is  likely  to  encounter. 

He  must  remember,  however,  that  even  the 
best  observatories,  equipped  as  they  are  with 
every  improved  type  of  instrument  and  with 
all  their  telegraphic  facilities,  are  sometimes 
very  much  out  in  their  forecasts,  so  that  he 
need  not  wonder  at  the  very  frequent  apparent 
failures  of  his  attempts. 

Wind,  which  is  simply  the  atmosphere  in 
motion,  is  of  two  kinds,  called  cyclonic  and 
anti-cyclonic. 

The  following  remarks  on  cyclones  and 
anti-cyclones  are  written  for  the  Northern 
Hemisphere,  and  to  apply  them  to  the  Southern 
Hemisphere,  all  directions  of  the  wind  round 
its  centre  should  be  reversed. 

67 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

A  cyclonic  wind  is  one  that  either  brings 
rain  or  is  associated  with  bad  weather. 

It  blows  spirally  round  a  centre  or  core  of 
low  pressure  in  a  direction  contrary  to  the 
hands  of  a  watch  in  the  Northern  Hemisphere. 
The  sequence  of  wind  and  weather  in  a  cyclone 
are  everywhere  the  same,  and  they  differ  in 
intensity  only  according  to  the  steepness  and 
closeness  together  of  the  isobars. 

The  following  definitions  should  be  learnt : 

Path  of  a  Storm. — Is  the  direction  that  the 
whole  storm  is  travelling  in. 

Trough  of  a  Storm. — Is  the  line  more  or 
less  at  right  angles  to  the  path  where  the 
barometer  has  reached  its  lowest  and  has 
just  turned  to  the  rise. 

Right  and  Left  Hand  Semicircles. — Are  the 
two  halves  of  the  storm  situated  on  the  right 
and  left  hand  respectively  of  the  observer, 
when  he  is  standing  in  the  centre  of  the  storm 
facing  the  direction  it  is  travelling  in. 

Centre  of  a  Storm. — Is  the  area  of  lowest 
pressure.  Here  the  wind  often  drops  to  a 
flat  calm. 

68 


METEOROLOGY 

Isobar. — Is  a  line  of  equal  barometric 
height  or  pressure. 

Isotherm. — Is  a  line  of  equal  temperature. 

The  wind  in  a  cyclonic  disturbance  does 
not  blow  tangentially  to  the  isobars,  but  spir- 
ally inwards  at  an  angle  of  about  io°-i5°  to 
them,  being  more  incurved  in  the  rear  part  of 
the  storm. 

In  the  Temperate  Zones  these  depressions 
almost  invariably  travel  eastwards,  but  their 
paths  may  be  deflected  by  land  or  by  an  area 
of  high  pressure. 

The  centre  of  a  storm  can  always  be 
found  by  the  following  rule,  known  as  Buys- 
Ballot's  Law. 

Rule. — Face  the  wind,  and  the  centre  will 
be  found  to  bear  about  135°  on  the  right  hand 
until  the  barometer  has  fallen  three-tenths  of 
an  ingh,  about  112°  between  three-tenths 
and  six-tenths,  and  about  90°  after  six- 
tenths. 

N.  B. — In  the  Southern  Hemisphere  it  will 
be  as  above,  but  on  the  observer's  left  hand. 

The  sketch  on  p.  70  shows  the  relation  of 
the  wind  to  the  isobars,  in  a  cyclonic  depres- 
sion, the  egg-shaped  lines  representing  lines  of 
equal  pressure. 


AIR  NAVIGATION   FOR   FLIGHT  OFFICERS 

It  must  be  clearly  understood  that  this 
sketch  is  purely  arbitrary,  and  that  a  cyclonic 
depression  may  take  any  shape  or  form  of 
isobar. 


FIG.  28. 

With  reference  to  the  statement  made 
before,  that  the  weather  sequence  in  a  cyclonic 
depression  was  always  the  same  but  differing 
in  intensity  only,  it  must  be  understood  that 
by  intensity  is  meant  that  whereas  in  one  case 
with  a  slight  and  gradual  fall,  which  means 

70 


METEOROLOGY 

that  the  isobars  are  spaced  wide  apart,  only 
a  mild  type  of  wind,  rain,  and  cloud  are 
experienced,  yet,  on  the  other  hand,  when  the 
isobars  are  close  together,  the  above  men- 
tioned are  met  with  in  a  much  greater  and 
stronger  form. 

As  a  general  rule,  the  steeper  the  fall  of  the 
barometer  the  stronger  the  wind  and  coming 
weather. 

Sometimes  it  will  be  noticed  that  a  big 
fall  of  the  barometer  is  not  attended  by 
any  drastic  change  in  the  weather,  but  that, 
after  a  time,  the  former  recovers  itself.  This 
is  due  to  what  is  known  as  '  Surge/ 

The  best  explanation  of  this  is  to  consider  a 
general  lowering  of  pressure  over  a  large  area, 
which  takes  some  time  to  fill  up  again,  the 
area  being  so  large  that  it  only  fills  up  com- 
paratively slowly. 

The  sketch  on  p.  72  shows  the  weather 
sequence  in  a  cyclonic  depression. 

The  rate  at  which  a  storm,  as  a  whole, 
travels  is  very  uncertain,  depending  on  the 
areas  of  high  pressure  round  it  and  the 
amount  of  land  about. 

Anti-cyclone. — This  is  a  region  of  high 
pressure  associated  with  fine  and  mild  weather, 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

in  which  the  wind  blows  more  or  less  tangenti- 
ally  to  the  isobars  with  the  hands  of  a  watch 
in  the  Northern  Hemisphere. 


Cirro-Stratus. 


Blue 

Refraction/ 
Detached, 


•Cumulus 

Hard/Sky 

Cool 

Showers\ 

*-» 
>r 

Squalls 


Detache 
Cumulu^ 


•Patches 
of 
lue 


Overcast 
StratcTcultrolus 


IiideFinec 
r    showers 


Drizz|e 
Driving  Rain 

^-     Dirty  Sky 

\ 


terySun 
dipped  Hills 


"Halo 


•ro-Stratus 


Strato-C/mulus 


Maceftails 


FIG.  29. 

Its  force  scarcely  ever  rises  above  a 
pleasant  breeze.  Unlike  a  cyclonic  depres- 
sion, an  anti-cyclone  may  remain  stationary 
for  days  on  end. 

One  great  feature  of  an  anti-cyclone  is  the 
radiation  weather  in  it. 


72 


METEOROLOGY 

The  sketch  below  gives  the  sequence 
of  weather  usually  experienced  in  an  anti- 
cyclone. 


FIG.   30. 

Formation    of   Cloud,  Fog,   and    Dew. — If 

the  barometric  pressure  at  any  place  falls, 
a  current  of  air  rises,  carrying  with  it  a  large 
amount  of  water  vapour,  more  especially  if 
the  low  pressure  should  happen  to  be  situated 
over  the  sea. 

73 


AIR   NAVIGATION   FOR  FLIGHT   OFFICERS 

As  this  air  rises,  it  expands  owing  to  the 
diminished  pressure  ;  this  causes  a  loss  of  heat, 
which  is  further  accentuated  by  the  low 
temperature  in  the  upper  regions.  This  loss 
of  heat  results  in  the  condensation  of  the 
water  vapour,  which  also  mixes  with  the  small 
particles  of  dust  and  other  matter  floating  in 
the  air. 

The  result  of  this  is  to  cause  the  familiar 
appearance  which  we  know  as  cloud. 

There  are  two  theories  that  have  been  put 
forward  as  to  the  formation  of  cloud. 

(1)  Is  known  as  condensation  by  cooling. 
This  method  has  been  described  above. 

(2)  Is  known  as  condensation  by  mixing. 
This  is  supposed  to  take  place  when  a  mass 
of  damp  air,  on  rising,  meets  another  mass 
of  damp  air  at  a  different  temperature. 

There  are  ten  different  classes  of  clouds, 
four  of  which  are  known  as  '  Fundamental 
Clouds/  whilst  the  other  six  are  made  up 
of  mixtures  of  the  other  four. 

The  following  tables  give  the  names  and 
average  heights  of  these  clouds. 

(a)  '  Fundamental  Clouds/ 

(1)  Stratus.          ...  o  to  3,500  feet. 

(2)  Nimbus,  or  Rain  Cloud  .       3,000    ,,  6,400     ,, 

74 


MARE'S    TAIL 

CIRRUS 

27000  to  50.000 ft. 


CIRRO-STRATUS 

Average  29.5OOft. 


IMALAYAS 

(KT  EVEREST) 


MACKEREL  SKY 

CIRRO-CUMULUS 

10,000  to  23,000 ft. 


ALTO-CUMULUS 

10.000  to  2  3,000  ft 


ANDES 

[ACONCAGUA) 


ALTO-STRATUS 

10,000  to  23,000ft 


STRATO-CUMULUS 

About  6,5OOffc. 


CUMULUS 

4.5OO  to  G.OOOft 


KITE    BOSTON 
-.US 


STORM   CLOUD 

CUMULO-NIMBUS 

4,5OO  to 


RAIIN    CLOUD 

NIMBUS 

3000  to  6,4-OOfl 


STRATUS 

O  to   3,5OOft 


ElFELTOWER 
ST.  PAULS 


THE    TEN    DIFFERENT    KINDS    OF    CLOUDS. 


METEOROLOGY 

(3)  Cumulus        .         .         .       4,500  to  6,000  feet. 

(4)  Cirrus,  or  Mare's  Tail     .     27,000  ,,  50,000     „ 

(b)  '  Composite   Clouds/ 

(1)  Cumulo  Nimbus,  or  Storm 

Cloud      .         .         .  4,5oo  to  24,000  feet. 

(2)  Strato  Cumulus              .  Average  6,500     ,, 

(3)  Alto  Stratus          .          .  10,000  to  23,000     ,, 

(4)  Alto  Cumulus       .          .  10,000   ,,  23,000     ,, 

(5)  Cirro  Cumulus,  or  Mac- 

kerel Sky         .         .     10,000  ,,  23,000     ,, 

(6)  Cirro  Stratus         .          .       Average  29,500     ,, 

The  accompanying  illustration  has  been 
published  through  the  courtesy  of  Mr.  Elliott 
Stock,  7  Paternoster  Row,  E.G.,  whose  per- 
mission has  been  obtained. 

Cause  of  Fog. — This  may  be  caused  in  two 
different  ways. 

(1)  Warm    air    saturated   with    moisture 
passing  over  a  cold  surface  of  water,   the 
vapour   in   the   air  is  chill   and  condensed, 
forming  a  white  cloud  called  fog. 

(2)  Cold   air   blowing   over  warm  water 
chills  the  water  vapour  rising  from  the  latter, 
with  the  same  result  as  in  the  first  case. 

A  fog  bank  may  be  driven  a  good  distance 
from  the  place  where  it  started,  provided  that 
the  air  temperatures  are  nearly  the  same ; 

75 


AIR  NAVIGATION   FOR   FLIGHT   OFFICERS 

but   such   fogs   do   not  last  long,  and  soon 
disappear. 

It  sometimes  happens  that  during  a  fog 
very  large  and  heavy  raindrops  come  down, 
this  is  a  sure  sign  that  the  fog  will  disappear 
very  shortly. 

General     Forecasting    of    Weather. — The 

general   forecasting   of   the   weather   of   the 
British  Islands  is  done  by  two  methods  : 

(1)  By  a  Synoptic  Analysis. 

(2)  By  Lord  Dunboyne's  method. 
Taking  the  two  in  the  sequence  mentioned 

above. 

(i)  By  Synoptic  Analysis. — At  7  A.M. 
every  morning  certain  information  is  tele- 
graphed to  the  headquarters  of  the  Meteoro- 
logical Office  from  all  stations  connected 
with  it,  and  also  wireless  reports  are  received 
from  ships. 

The  information  thus  received  is  collated 
and  placed  on  the  weather  chart  for  the  day, 
ready  for  issue.  The  information  telegraphed 
to  the  central  office  is  as  follows : 

Force  and  direction  of  wind. 

Height  of  barometer,  and  whether  rising 
or  falling, 

76 


METEOROLOGY 

Temperature  of  air  and  sea,  the  latter 
only  at  those  stations  bordering  the  coast. 

State  of  weather  prevailing  at  the  time 
at  each  station. 

State  of  sea. 

These  observations  are  placed  on  the  chart 
as  necessary  ready  for  issue  to  the  general 
public,  though  this  has  been  modified  during 
the  war  by  issue  only  to  official  bodies. 

The  symbols  in  use  on  the  synoptic  chart 
are  given  below. 

Isobars. — Are  denoted  by  continuous  lines. 
Isotherms. — Are  denoted  by  pecked  lines. 

Wind  force  is  denoted  as  shown  below ; 
the  direction  of  the  wind  goes  with  the  arrows, 
and  is  named  according  to  where  it  comes 
from. 


® 


Force  above  10. 
Force  8-10. 
Force  4-7. 
Force  1-3. 
Calm. 


FIG.  31. 

77 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 

The  general  state  of  the  weather  is  shown 
below. 

•          Rain. 
A         Hail. 

Snow. 

^        Fog. 
T          Thunder. 
K          Thunderstorm] 

Rough  Sea. 

=£===:  High  Sea. 

Wireless  Report. 

FIG.  32. 

(2)  Dunboyne's  Weather  Report.  —  This 
report  is  issued  by  the  Admiralty  daily  at 
10.30  A.M.  It  is  liable  to  revision  as  time 
goes  on,  and  actual  observation  shows  the 
need  for  it. 

The  weather  report  is  for  the  British 
Islands  in  general  and  London  in  particular. 

It  divides  the  British  Islands  into  three 
parts  as  follows  : 

(1)  North  of  the  latitude  of  the  Wash. 

(2)  The     English     Channel     and     north 
coast  of  France. 

(3)  The     southern     halves     of     England 

78 


METEOROLOGY 

and  Ireland  south  of  the  latitude  mentioned 
in  (i). 

On  the  daily  sheet  is  printed  an  explana- 
of  the  terms  used,  as  follows  : 

The  day  referred  to  is  a  twenty-four  hour 
day. 

Fine. — The  wind  moderate  in  force  or 
less,  no  appreciable  rainfall,  probably  some 
hours  of  sunshine. 

Fair. — The  wind  fresh  in  force  or  less, 
little  or  no  rain,  probably  cloudy. 

Changeable. — Sometimes  fair  or  fine,  some- 
times unsettled. 

Unsettled. — A  high  wind  alone  or  heavy 
rain  alone,  or  both  wind  and  rain  combined 
in  moderation. 

Disturbed. — High  wind  or  gale  with  rain 
more  or  less.  On  some  occasions  the  term 
'  Very  Disturbed  '  may  be  used. 

N.B. — Intervals  of  fog  may  occur  during 
the  periods  of  fair  or  fine. 

Period. — Four  days  or  more. 

Interval. — Twelve  hours  or  less. 

Spell. — More  than  twelve  hours,  less  than 
four  days. 


CHAPTER  VII 

GENERAL  WEATHER  IN  THE 
BRITISH  ISLANDS 

THE  prevailing  wind  in  the  British  Islands  is 
from  some  westerly  point. 

Two  of  the  principal  reasons  are  as 
follows  : 

(1)  The  British  Islands,  situated  as  they 
are  in  a  high  northern  latitude,  are  in   the 
region  of  the  '  Anti-trades  '  or  Westerlies. 

(2)  There    is     usually     a    low    pressure 
round  about  Iceland,  and   a  high  pressure 
about  the  Azores,  and,  bearing  in  mind  the 
direction   of  the   wind   circulation   round   a 
high  and  low  pressure  respectively,  the  result 
is  as  shown  in  the  sketch  on  p.  81. 

Much  could  be  said  about  the  cause  of 
wind  due  to  the  earth's  rotation,  but  it  is 
not  proposed  to  touch  on  this  in  these  notes. 
(See  Appendix.) 

Should  the  reader  require  to  go  further 
into  this  matter,  he  should  consult  the 
Admiralty  '  Manual  of  Navigation/ 

80 


WEATHER  IN  BRITISH  ISLANDS 

Westerly  gales  are  very  prevalent  in  the 
winter  months,  i.e.  from  October  to  March 
inclusive  ;  they  are  rare  from  May  to  July, 
also  inclusive,  and  seldom  last  lorg. 

In  the  English  Channel,  winds  from  N.N.E. 
to  E.  cause  the  land  to  become  covered  with 
a  thick  white  fog  resembling  smoke. 

•  Iceland 


•  British  Islands 

•  Azores 
FIG.  33. 

Easterly  winds  are  very  common  in  the 
spring  months.  A  south-easterly  wind  with 
a  falling  barometer  is  an  almost  infallible  sign 
of  a  coming  gale. 

Land  and  sea  breezes  may  occur  during 
a  long  spell  of  fine  weather,  the  land  breeze 
by  night  and  the  sea  breeze  by  day. 

Long-drawn-out  calms  are  suspicious,  and 
81  G 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

are  generally  the  advance  guard  of  a  spell  of 
bad  weather. 

The  paths  of  storms  passing  over  the 
British  Islands  are  rather  erratic,  owing  to 
their  being  deflected  by  the  land.  They  may 
also  be  deflected  by  coming  up  against  a  high- 
pressure  system. 

Storms  passing  over  the  British  Islands 
almost  always  have  their  centres  north  of 
the  English  Channel ;  from  this,  reference  to 
Fig.  28  will  show  that  the  usual  dangerous 
wind  will  be  south-easterly,  with,  of  course, 
the  barometer  falling. 

Storm  Signals. — These  are  hoisted  by  the 
various  storm  signal  stations  according  to 
orders  received  from  the  Meteorological  Office, 
from  the  warnings  given  by  their  synoptic 
charts.  The  new  system  is  known  as  the 
International  Code,  but  its  introduction  has 
been  delayed  by  the  war. 

It  consists  of  a  display  of  either  one  or  two 
cones  hoisted  as  follows  : 

One  Cone,  point  upwards.- — Gale  commencing  with 
wind  in  the  north-west  quadrant. 

One  Cone,  point  downwards. — Gale  commencing 
with  wind  in  the  south-west  quadrant. 

Two  Cones,  one  above  the  other,  both  points 
82 


BEAUFORT'S  SCALES 


upwards. — Gale  commencing  with  the  wind  in  the 
north-east  quadrant. 

Two  Cones,  one  above  the  other,  both  points  down- 
wards.— Gale  commencing  with  the  wind  in  the 
south-east  quadrant. 

Two  Cones,  bases  together. — Hurricane.  Wind 
force,  12  Beaufort  Scale. 

Beaufort's  System  of  Weather  Notation. — 
The  following  tables  have  been  copied  from 
the  Admiralty  '  Manual  of  Navigation/  per- 
mission to  do  so  having  been  given  by  the 
Controller  of  H.M.  Stationery  Office. 

Beaufort's  System  of  Wind  Notation 

For  Coast  Use. 

Calm  :  smoke  rises 
vertically 

Direction  of  wind 
shown  by  smoke  drift 
but  not  by  wind  vane 
Wind  felt  on  face, 
leaves  begin  to  rustle, 
ordinary  vane  moved 
by  wind 

Leaves  and  twigs 
in  constant  motion, 
wind  extends  a  light 
flag 

Raises  dust  and 
loose  paper,  small 
branches  are  moved 
Small  trees  in  leaf  be- 
gin to  sway,  crested 
wavelets  form  on  in- 
land waters 

83 


No. 
0 

General  Description 

Calm 

I 

Light  air 

2 

Slight  breeze 

3 

Gentle  breeze 

4 

Moderate 
breeze 

5 

Fresh  breeze 

Miles  per 
Hour. 

Less 

Metres  per 
Second.  < 

Less  than 

than  i 

0-3 

4-7 

1-6-3-3 

8-12 

3,-5, 

13-18 

5-5-8-0 

19-24 

8-i-io*7 

AIR  NAVIGATION    FOR   FLIGHT   OFFICERS 


b 

be 

c 

o 

g 

m 
f 


For  Coast  Use. 
Large    branches    in 
motion,        whistling 
heard    in    telegraph 
wires 

Whole  trees  in  mo- 
tion,    inconvenience 
felt    when    walking 
against  wind 
Breaks  twigs  off  trees, 
generally       impedes 
progress 

Slight  structural  dam- 
age occurs,  chimney- 
pots and  slates  re- 
moved 

Seldom  experienced 
inland,  trees  up- 
rooted, considerable 
structural  damage 
occurs 

Very  rarely  experi- 
enced, causes  wide- 
spread damage 


Beaufort's  System  of  Weather  Notation 

Blue  sky,  i.e.  sky  not  more  than  J  clouded. 

Sky  I  to  |  clouded. 

Sky  J  to  f  clouded. 

Sky  overcast,  i.e.  more  than  £  clouded. 

Gloomy, 

Mist. 

Fog. 

84 


No. 

6 

General  Description. 

Strong  breeze 

7 

High  wind 

8 

Gale 

9 

Strong  gale 

10 

Whole  gale 

ii 

Storm 

12 

Hurricane 

Miles  per 
Hour. 

25-31 

Metres  per 
Second. 
IO-8-I3'8 

32-3» 

13.9-17-1 

39-46 

iy-2-20'7 

47-54 

20-8-24-4 

55-63 

24-5-28-4 

64-75 

28-5-33'5 

Above 
75 

33-6  and 
above 

BEAUFORT'S  SCALES 

r  Rain, 

d  Drizzling  rain, 

e  Wet  air  without  rain  falling, 

p  Passing  showers, 

h  Hail. 

s  Snow, 

t  Thunder. 

1  Lightning, 

tl  Thunderstorm. 

tlr  Thunderstorm  accompanied  by  rain, 

q  Squalls. 

u  Ugly  threatening  sky. 

v  More  than  ordinary  visibility. 

w  Unusually  heavy  dew. 

x  Hoar  frost, 

z  Dust  haze  or  smoke. 

Beaufort's  Scale  for  Sea  Disturbance 


No. 
O 

Description. 
Calm. 

I 

Very  smooth. 

2 

Smooth. 

3 

Slight. 

4 

Moderate. 

5 

Rather  rough. 

6 

Rough. 

7 

High. 

8 

Very  high. 

9 

Phenomenal. 

85 

CHAPTER  VIII 

FORECASTING  BY  SOLITARY 
OBSERVER 

IN  connection  with  this,  the  solitary  observer 
has  the  following  information  at  his  disposal. 

(1)  The  ordinary  Daily  Weather  Notice, 
from  which  he  can  obtain  the  positions  of  the 
high  pressures.     This  gives  him  the  probable 
path  of  any  low  pressure. 

(2)  His  knowledge  from  personal  observa- 
tion of  the  present  state  of  the  weather. 

(3)  The  movements  of  the  barometer  from 
his  record,  or  from  the  trace  shown  by  his 
barograph. 

(4)  The    wireless    reports    received    from 
stations  or  ships  to  the  westward  of  him, 
bearing  in  mind  that  nearly  all  depressions, 
with  their  attendant  bad  weather,  are  travel- 
ling to  the  eastward. 

With  reference  to  the  trace  shown  by  the 
barograph,  it  should  be  remembered  that, 
should  the  fall  of  the  barometer  be  at  a  uni- 
form rate,  the  trace  on  the  paper  will  be  a 
descending  straight  line ;  if  the  rate  of  fall  is 

86 


FORECASTING  WEATHER 

increasing,  the  trace  becomes  convex,  and  if 
the  rate  is  decreasing,  the  trace  is  concave  to 
the  top  of  the  recording  sheet. 

If  the  rise  of  the  barometer  is  at  a  uniform 
rate,  the  trace  is  shown  by  an  ascending 
straight  line ;  if  the  rate  is  increasing,  the 
trace  is  concave ;  whilst  if  it  is  decreasing,  the 
trace  is  convex  to  the  top  of  the  recording 
sheet. 

Thus  all  we  can  tell  from  the  movements 
of  the  barograph  is  that,  with  a  falling  glass, 
a  convex  trace  means  that  the  wind  and 
weather  will  get  worse  much  more  rapidly 
than  with  a  concave  trace,  and  with  a  rising 
glass,  a  concave  trace  will  indicate  that  the 
weather  will  improve  more  rapidly  than  with 
a  convex  trace. 

On  the  other  hand,  the  quicker  the  rise  or 
fall,  the  steeper  the  isobars,  and  therefore  the 
stronger  the  wind. 

The  above  is  shown  by  the  diagrams  on 
p.  88. 

Bad  weather,  which  takes  a  long  time  to 
develop,  is  also  long  in  showing  improve- 
ment, and  vice  versa. 

This  can  easily  be  remembered  by  the  old- 
time  jingle  : 

Long  foretold,  long  last, 
Short  notice,  soon  past. 


I.  Steady  Rate  of  Fall  or  Rise. 

Hrs.i 


Barer 
30 

neter 
00 

/ 

X 

J 

^ 

29 

50 

« 

#% 

& 

^     29 

•00 

£* 

28 

i  ' 
50 

II.  Increasing  Rate  of  Fall  or  Rise. 
Hrs.I        n      m      JYYYIMYfflJXXH 


Barometer 


30 


00 


29 


00 


50 


28  00 


III.  Decreasing  Rate  of  Fall  or  Rise. 
HrsI        H      ffl      W    .Y      ¥J      YJ    YJH      K      X      XI      Xtt      I 


WEATHER  RULES 

In  connection  with  the  forecasting  of  the 
coming  weather,  the  rules  given  by  the  late 
Admiral  Fitzroy  are  well  worth  committing 
to  memory. 

Taken  in  conjunction  with  other  instru- 
mental aids,  they  are  of  the  greatest  use  in 
foretelling  weather. 

These  rules  are  given  below. 

Admiral      Fitzroy's     Weather      Rules.  — 

Whether  clear  or  otherwise,  a  rosy  sky  at 
sunset  indicates  fine  weather  ;  a  sickly  green- 
ish hue,  wind  and  rain  ;  tawny  or  coppery 
clouds,  wind  ;  a  dark  or  Indian  red,  rain  ; 
a  red  sky  in  the  morning,  bad  weather,  or 
much  wind,  perhaps  also  rain  ;  a  grey  sky 
in  the  morning,  fine  weather  ;  a  high  dawn, 
wind  ;  a  low  dawn,  fine  weather. 

The  darker  or  angrier  the  colour  of  the 
red  in  the  morning,  the  worse  the  coming  bad 
weather  will  prove  to  be.  Also  an  opal-tinted 
sky  in  the  morning  is  a  sign  of  coming  bad 
weather. 

A  high  dawn  is  when  the  first  indications  of 
daylight  are  seen  above  a  bank  of  clouds. 

A  low  dawn  is  when  the  day  breaks  on  or 
near  the  horizon,  the  first  streaks  of  light  being 
very  low  down. 

89 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

Soft-looking  or  delicate  clouds  foretell 
fine  weather,  with  moderate  or  light  winds ; 
hard-edged  oily  looking  clouds  show  wind. 
A  dark,  gloomy  blue  sky  is  windy ;  but  a  light, 
bright  blue  sky  indicates  fine  weather. 

Generally,  the  softer  clouds  look,  the  less 
wind  but  perhaps  more  rain  may  be  expected ; 
and  the  harder,  more  greasy,  rolled,  tufted 
or  ragged,  the  stronger  the  coming  wind 
will  prove  to  be. 

A  bright  yellow  sky  at  sunset  foretells 
wind  ;  a  pale  yellow,  rain  ;  orange  or  copper- 
coloured,  wind  and  rain  ;  and  thus,  by  the 
prevalence  of  the  various  tints  in  the  sky, 
the  coming  weather  may  be  foretold  fairly 
accurately,  and,  if  aided  with  the  usual 
instruments,  almost  exactly.  Light  delicate 
quiet  tints  or  colours,  with  soft  indefinite 
forms  of  clouds,  indicate  and  accompany  fine 
weather,  but  gaudy  or  unusual  hues,  with 
hard  definitely  outlined  clouds,  foretell  rain 
and  probably  strong  wind. 

Small  inky-looking  clouds  foretell  rain  ; 
light  scud  clouds  driving  across  heavy  masses, 
show  wind  and  rain ;  but  if  alone,  may 
indicate  wind  only,  the  latter  proportionate 
to  their  motion. 

High  upper  clouds  crossing  in  a  direction 
90 


WEATHER  RULES 

different  from  that  of  the  lower  clouds,  or 
from  the  surface  wind  felt  below,  foretell  a 
change  toward  their  direction. 

After  fine  clear  weather,  the  first  signs  in 
the  sky  of  a  coming  change. are  usually  light 
streaks,  curls,  wisps,  or  mottled  patches  of 
distant  cloud,  which  increase  and  are  followed 
by  a  general  overcasting  of  vapour  that  grows 
into  cloudiness. 

This  appearance,  more  or  less  oily  or 
watery,  as  rain  or  wind  will  predominate, 
is  a  certain  sign. 

Usually,  the  higher  and  more  distant  such 
clouds  seem  to  be,  the  more  gradual  but  more 
general  the  coming  change  of  weather  will 
prove  to  be. 

Misty  clouds  forming  or  hanging  on  heights 
show  wind  and  rain  approaching;  if  they 
remain,  increase  or  descend.  If  they  rise  or 
disperse,  the  weather  will  get  better  or  become 
fine. 

Dew  is  an  indication  of  fine  weather,  its 
formation  never  begins  under  an  overcast 
sky  or  when  there  is  much  wind.  Great 
clearness  of  the  air,  especially  near  the 
horizon,  distant  objects  very  well  defined  or 
raised  by  refraction,  also  what  is  called  a  good 
hearing  day,  are  signs  of  rain  or  wind  coming. 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

A  great  deal  of  refraction  is  a  sign  of 
easterly  wind. 

More  than  usual  twinkling  or  apparent 
size  of  the  stars,  haloes,  etc.,  are  more  or  less 
indications  of  approaching  wind,  with  or 
without  rain. 


92 


CHAPTER  IX 
ASTRONOMY 

IN  olden  days  the  sky  and  its  stars  were 
divided  into  twelve  constellations. 

These  constellations  were  supposed  to 
represent  human  beings  and  different  animals. 

After  telescopes  were  invented,  and  as  the 
power  of  the  latter  grew,  more  and  more 
stars  became  visible,  and  the  original  twelve 
constellations  outgrew  themselves. 

In  modern  star  maps,  this  number  twelve 
has  been  greatly  increased,  and  in  those  drawn 
by  the  late  Mr.  R.  A.  Procter,  no  less  than 
eighty-four  constellations  are  given. 

Some  of  the  latter  are  very  small  and 
do  not  contain  any  stars  which  would  be 
of  practical  value  to  the  pilot,  and  in  the 
following  star  maps,  twenty-two  in  number, 
only  those  constellations  are  given  which 
might  be  of  use  to  a  Flight  Officer. 

These  drawings  only  give  the  principal 
stars  in  each  of  the  constellations ;  of  course 

93 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 

there  are  many  more,  but  it  would  serve  no 
good  purpose  by  putting  them  in,  and  would 
only  lead  to  confusion. 

If  any  more  stars  are  required  by  the  pilot, 
he  cannot  do  better  than  consult  Procter's 
Star  Atlas. 

The  stars  in  the  following  drawings  are 
not  put  in  exactly  correct  as  regards  their 
decimations  and  right  ascensions,  but  they 
are  near  enough  for  all  practical  purposes. 

As  the  stars  in  the  constellations  are 
lettered  according  to  the  Greek  alphabet,  the 
latter  is  here  appended  for  the  benefit  of  those 
who  may  not  know  it. 

a  Alpha.  v  Nu. 

£  Beta.  f  Xi. 

y  Gamma.  o    Omicron. 

8  Delta.  IT  Pi. 

e  Epsilon.  p  Rho. 

£  Zeta.  cr  Sigma. 

??  Eta.  T  Tau. 

6  Theta.  v  Upsilon. 

i  Iota.  <£  Phi. 

K  Kappa.  %  Chi. 

\  Lambda  ^  Psi 

fju  Mu.  a)  Omega. 

These  Greek  letters  are  given  against  each 
star  in  the  sketches,  and  also  the  old  Arabic 

94 


ASTRONOMY 

names  in  the  case  of  the  more  important  ones 
in  each  constellation. 

In  these  drawings  the  true  north  should  be 
taken  as  the  top  of  the  page. 

Owing  to  their  immense  distance  .away, 
the  relative  position  of  the  stars  to  one  another 
as  seen  from  the  earth  seems  to  be  always  the 
same,  but  as  a  matter  of  fact  they  all  undergo 
a  slight  change  every  year  in  the  same  direc- 
tion, known  as  '  Precession/  This  does  not, 
of  course,  alter  their  relative  positions  to  one 
another.  So  that,  having  once  picked  up  a 
star  with  reference  to  its  relative  position  to 
another  constellation,  it  will  always  be  found 
in  that  same  place. 

On  account  of  the  diurnal  motion  of  the 
earth,  the  Compass  Bearing  of  any  star  is 
always  changing  from  the  time  it  rises  to  the 
time  it  sets. 

When  looking  for  stars  at  night,  it  often 
happens  that  the  constellations  they  are  in 
may  be  upside  down. 

This  is  due  to  the  apparent  rotation  of  the 
Stellar  Sphere,  which  appears  to  revolve  from 
east  to  west  round  the  axis  of  the  earth. 

Several  of  these  constellations  are  what  is 
known  as  '  Circumpolar/  that  is  to  say,  they 
never  set  in  these  latitudes. 

95 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

The  Great  and  Little  Bears  and  Cassiopeia 
are  examples  of  this. 

A  sketch  is  given  to  illustrate  this 
paragraph. 


Hour  Angle 


HourAnqle  * 


*PoleStar 


HourAnqle 
6K  oom  oo* 


HourAnqle 
oo h  oomoo5 


FIG.  35. 

Note. — The  hour  angle  referred  to  is  the 
hour  angle  of  a  (Dubhe)  Ursa  Majoris. 

The  pole  star  or  '  Polaris  '  is  situated  very 
nearly  at  the  north  pole  of  the  celestial 
concave,  revolving  round  it  about  i J  degrees 

96 


CONSTELLATIONS 

from  it.  It  can  be  found  by  drawing  a  line 
from  a  and  &  Ursa  Majoris,  and  continuing 
it  towards  the  pole. 

When  looking  for  a  star,  it  should  be 
remembered  that  if  the  declination  of  the 
star  is  less  than  (or  south  of)  the  observer's 


/Alioth 


*Megrez  *Dubhe 


*Phecda  *Merak 


v  Talithi 


FIG.  36. — Ursa  Majoris.      (The  Great  Bear.) 

latitude,  it  will  cross  the  meridian  south  of 
him ;  if  equal  to  the  latitude,  it  will  rise  due 
east,  pass  directly  overhead,  and  set  due  west ; 
if  greater  than  (or  north  of)  the  observer's 
latitude,  it  will  always  be  north  of  him. 

For  a  beginner,  the  best  constellations  to 
learn  in  order  to  connect  up  the  other  big 
stars,  are  the  Great  Bear  and  Orion. 

97  H 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 


FIG.  37. — Cassiopeia.     (The  Chair.) 


y 

*Alrnach 


*Mirach 


*Alpheratz 


*Scheai 


FIG.  38. — Square  of  Pegasus. 

98 


*Markab 


CONSTELLATIONS 


if  I'araied 

#Altair 
^        I**  Mag 

*Alshain 


FIG.  39.— Aquila.     (The  Eagle.) 


FIG.  40. — Aries.     (The  Ram. 


99 


FIG.  41.— Auriga.     (The  Charioteer.) 


*Nekkar 


^Arcturus 
Is1  Maq. 


*Muphrid 


FIG.  42. — Bootes.     (The  Herdsman.) 
100 


CONSTELLATIONS 


FIG.  43. — C  arris  Major.     (The  Greater  Dog. 


' 

*Gomeisa 


# Procyon 
\*.  Mag. 


FIG.  44. — Canis  Minor.     (The 
Lesser  Dog.) 

101 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 


FIG.  45.— Catus.     (The  Whale.] 


FIG.  46. — Corona  Borealis.     (The 
Northern  Crown. 

102 


CONSTELLATIONS 


Jta  The  Centaurs 

Ist  Maq. 


*a 

^  Mag 


FIG.  47. — Crux.     (The  Southern  Cross.) 


FIG.  48. — Cygnus.     (The  Swan.) 
103 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 


*  Castor 


*Pollux 


S 
*Wasat 


*Mebsuta 


y 

*Alhena 


FIG.  49. — Gemini.     (The  Twins.) 


*Zosma 


*Deneb 


Ist  Mag. 


FIG,  50, — Leo.     (The  Lion.) 
104 


CONSTELLATIONS 


^Beteleux 


*Bellatrix 


*Vega 
Ist  Mag. 


*Sheliak 


Sulaphat 


FIG.  51. — Lyra.     (The  Lyre.) 


*Alnilam 


TheGreat  Nebula 


*Kaph 


FIG.  52. — Orion.     (The  Hunter.) 


*Scheat 


*  Alqenib 


a 
*Markab 


*Homan 


FIG.  53. — Pegasus.     (The  Winged  Horse.) 
105 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 


*Nath 


Aldebaran 


FIG.  54.— Taurus.     (The  Bull.] 


The  Pleiades 
?** 
Alcyone 


FIG.  55.— Ursa  Minor.     (The  Little  Bear.) 

106 


FIG.  56. — Scorpio.     (The  Scorpion, 


FIG;  57. — Corvus.     (The  Crow.) 
107 


AIR   NAVIGATION   FOR   FLIGHT  OFFICERS 


*  Square  of  Peasus 


Schedar 
*    *  Cassiopeia 


Corona  Borealis 


Alphacca 


*  Pole  Star 


*  Little  Bear 


*  Great  Bear 


Arcturus      Algeiba** 


*  Requlus 


Menkalman 
^Caoella 


FIG.  58.— The  Great  Bear  and  the  Stars  it  leads  to. 

108 


CONSTELLATIONS 


*Castor 
*Pollux 


*Sirius 


*Canopus 


*Nath 

Aldebaran         Pleiades 
^  Alcyone 


FIG.  59. — Orion  and  the  Stars  it  leads  to. 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

Before  entering  into  the  problems  con- 
nected with  the  sun  and  moon,  it  would  be 
as  well  to  give  some  explanation  of  the  various 
terms  used. 

The  reader  should  make  himself  acquainted 
with  the  following  definitions  in  Nautical 
Astronomy : 

Definitions. —  A  Sphere. — A  sphere  is  a 
solid  bounded  by  a  surface,  every  point  of 
which  is  equally  distant  from  a  fixed  point 
called  the  centre. 

A  Great  Circle. — A  great  circle  is  a  section 
of  the  surface  of  a  sphere,  made  by  a  plane 
passing  through  the  centre. 

A  Small  Circle. — A  small  circle  is  a  section 
of  the  surface  of  a  sphere,  made  by  a  plane  not 
passing  through  the  centre. 

Earth's  Axis. — The  axis  of  the  earth  is 
the  diameter  about  which  it  revolves  with  a 
uniform  motion  from  west  to  east. 

Earth's  Poles. — The  poles  of  the  earth  are 
the  extremities  of  its  axis. 

Equator. — Is  the  great  circle  whose  axis 
and  poles  are  the  axis  and  poles  of  the 
earth. 

Meridians. — Are  great  circles  whose  planes 
pass  through  the  poles  of  the  earth. 

no 


DEFINITIONS 

Meridian  of  a  Place. — Is  that  meridian 
which  passes  through  the  place. 

Prime  Meridian. — Is  that  fixed  meridian 
by  reference  to  which  the  longitudes  of  all 
other  places  on  the  earth  are  measured. 

Parallels  of  Latitude. — Are  small  circles 
whose  planes  are  parallel  to  the  plane  of  the 
equator. 

Longitude  of  a  Place. — Is  the  smaller  arc  of 
the  equator,  intercepted  between  the  prime 
meridian  and  the  meridian  passing  through 
the  place. 

Latitude  of  a  Place. — Is  the  arc  of  a 
meridian  intercepted  behind  the  equator 
and  the  place. 

Difference  of  Latitude  between  two  Places. — 
Is  the  arc  of  a  meridian  intercepted  between 
their  parallels. 

Difference  of  Longitude  between  two  Places. 
— Is  the  smaller  arc  of  the  equator  intercepted 
between  their  meridians. 

Celestial  Concave. — Is  the  interior  surface 
of  a  globe  bounded  by  the  blue  of  space,  and 
on  which  all  the  heavenly  bodies  appear  to  be 
situated. 

Poles  of  the  Heavens. — Are  the  points 
where  the  earth's  axis  produced,  cuts  the 
celestial  concave. 

in 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 

Ecliptic. — Is  the  apparent  path  of  the  sun 
during  the  year  on  the  celestial  concave. 

Equinoctial  or  Celestial  Equator. — Is  the 
great  circle  formed  by  the  plane  of  the  earth's 
equator  produced,  cutting  the  celestial 
concave. 

The  Equinoctial  Points. — Are  the  two 
points  on  the  celestial  concave  where  the 
ecliptic  and  the  equinoctial  cut  one  another. 
One  is  known  as  the  First  Point  of  Aries 
(the  point  on  the  ecliptic  where  the  sun's 
declination  changes  from  south  to  north), 
the  other  as  the  First  Point  of  Libra  (the 
point  on  the  ecliptic  where  the  sun's  declina- 
tion changes  from  north  to  south). 

Circles  of  Declination. — Are  great  circles 
which  pass  through  the  poles  of  the  heavens ; 
they  correspond  to  terrestrial  meridians. 

Parallels  of  Declination. — Are  small  circles 
whose  planes  are  parallel  to  the  plane  of  the 
equinoctial. 

Declination. — Is  the  arc  of  a  circle  of 
declination  intercepted  between  the  equi- 
noctial and  the  place  of  the  body.  It  is 
thus  similar  to  latitude  on  the  earth.  It  is 
measured  north  and  south  of  the  equinoctial 
from  o  at  the  equinoctial  to  90°  at  each 
celestial  pole. 

112 


DEFINITIONS  AND  TIME 

Polar  Distance  of  a  Heavenly  Body. — Is  the 
arc  of  a  circle  of  declination  through  the  body 
intercepted  between  the  elevated  pole  and 
the  body,  and  is  therefore  (90° -dec.)  or 
(90°  +  dec.)  according  as  the  declination  is 
of  the  same  or  opposite  name  to  the  latitude. 

N.  B.~ The  elevated  pole  is  that  one 
situated  in  the  same  latitude  as  the  observer. 

Right  Ascension. — Is  the  arc  of  the  equi- 
noctial intercepted  between  the  First  Point  of 
Aries  and  the  Circle  of  Declination  which 
passes  through  the  body,  measured  anti- 
clockwise from  oh  to  24h. 

Notes  on  Time.— As  time  plays  a  very 
important  role  in  the  sun  and  moon  problems, 
a  few  notes  on  the  subject  are  given  here 
before  going  into  the  problems. 

This  should  be  thoroughly  studied  and 
understood ;  by  doing  so,  half  the  difficulty  of 
working  out  the  problems  is  done  away  with 
—in  fact  more  than  half. 

Time  may  be  divided  into  two  sorts — Civil 
and  Astronomical. 

Civil  Time  is  divided  into  two  periods 
called  A.M.  (ante  meridiem],  and  P.M.  (post 
meridiem). 

Each  of  these  is  a  period  of  twelve  hours, 
113  i 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

the  A.M.  time  being  from  midnight  to  noon, 
and  the  P.M.  time  from  noon  to  midnight. 


FIG.  60. — The  Celestial  Concave. 

ECKQ  is  the  equinoctial. 

FBHG  is  a  parallel  of  declination. 

ABCD  and  AHKD  are  circles  of  declination. 

LYM  is  the  ecliptic — Y  the  First  Point  of  Aries. 

HK  is  the  declination  of  the  body  H. 

YK  is  the  right  ascension  of  the  body  H. 

H4 


TIME 

The  civil  day  and  date  commences  at 
midnight  and  ends  the  following  midnight. 

Astronomical  Time  is  reckoned  in  one 
period  of  twenty-four  hours,  the  day  and  date 
commencing  at  noon  and  changing  the  follow- 
ing noon. 

From  this  it  will  be  seen  that  the  civil  date 
is  always  twelve  hours  ahead  of  the  astro- 
nomical date,  i.e.  the  former  begins  at  mid- 
night and  the  latter  the  following  noon. 

When  working  problems  in  time,  it  must 
be  remembered  that  twenty-four  hours  can 
always  be  added  to  any  time,  provided  that 
the  date  is  placed  one  day  back. 

Examples  : 

4h   oom   oos  on  May  4,  can,    if   necessary,    be 
shown  as  28h  oom  oo«  on  May  3. 

2oh  oo  oos  on  June  17,  can  be  shown  as 
44h  oom  oos  on  June  16. 

Civil  time  can  always  be  converted  into 
astronomical  time,  and  vice  versa,  remembering 
that  civil  date  is  always  twelve  hours  ahead 
of  astronomical  date. 

Examples : 

Civil  time  4  A.M.  March  30. 

Astronomical  time,  i6h  oom  oos  March  29. 


AIR    NAVIGATION   FOR  FLIGHT  OFFICERS 

Civil  time,  n    P.M.  March  30. 
Astronomical  time,  nh  oom  oos  March  30. 

Civil  time,  H&  A.M.  March  30. 
Astronomical  time,  23h  oom  oos  March  29. 

From  No.  2  of  the  above  examples  it  will  be 
noticed  that  civil  time  and  astronomical  time 
are  identical  in  date  during  P.M.  civil  time,  but 
whilst  the  civil  date  changes  at  midnight,  the 
astronomical  date  goes  on  for  another  twelve 
hours. 

Examples : 

Astronomical  time,  15^  oom  oos  July  12. 
Civil  time,  3h  A.M.  July  13. 

Astronomical  time,  ioJ»  oom  oo8  July  12. 
Civil  time,  lot  P.M.  July  12. 

Time  is  divided  into  two  kinds— Apparent 
Solar  Time  and  Mean  Solar  Time. 

Apparent  Time.- — Is  the  actual  time  shown  by 
the  sun,  but  owing  to  the  elliptical  shape  of  the 
earth's  orbit,  the  apparent  proper  motion  of  the  sun 
is  not  uniform,  so  that  the  apparent  solar  day,  hour, 
minute,  and  second  are  not  quite  of  constant  length. 

Mean  Sun.- — Is  an  imaginary  sun  which  moves 
in  the  equinoctial  with  the  apparent  sun's  mean 
motion  in  R.A. 

Mean  Time. — This  is  the  time  shown  by  an 
116 


EFFECT  OF  LONGITUDE  ON  TIME 

imaginary  sun  whose  motion  is  uniform  in  velocity, 
along  the  ecliptic,  and  to  which  our  clocks  are  set. 

The  velocity  of  the  apparent  sun  not 
being  uniform,  it  follows  that  it  will  be  some- 
times ahead  of  the  mean  sun  and  sometimes 
behind  it. 

This  difference  is  called  the  '  Equation  of 
Time/  and  is  given  in  the  Nautical  Almanac 
for  every  two  hours  of  the  day  throughout  the 
year.  In  problems  connected  with  the  sun's 
bearing,  the  times  given  in  the  true  bearing 
or  azimuth  tables  are  all  apparent  times, 
so  that  it  is  necessary  to  change  the  time  by 
watch  into  apparent  time. 

The    Effect    of    Longitude. — The  way    in 

which  the  longitude  of  a  place  on  the  earth's 
surface  affects  the  time  of  that  place,  should 
be  clearly  understood,  as  it  helps  to  give  one 
a  firm  grasp  on  the  problems  later  on. 

The  earth  revolves  from  west  to  east  in 
twenty-four  hours,  but  it  is  more  convenient 
to  imagine  the  earth  as  stationary,  and  the 
celestial  concave  revolving  from  east  to  west 
about  its  own  axis. 

This  comes  to  the  same  thing. 

Consequently,  as  we  measure  our  noon  by 
the  sun's  passage  across  the  meridian  of  our 

117 


AIR  NAVIGATION   FOR   FLIGHT  OFFICERS 

place,  it  follows  that  the  sun  must  have 
already  crossed  the  meridian  of  any  place  to 
the  eastward  of  our  position,  and  not  yet 
crossed  the  meridian  of  those  places  to  the 
westward  of  us. 

In  all  our  charts,  the  meridian  passing 
through  the  transit  instrument  at  Greenwich 
Observatory  is  taken  as  the  prime  meridian, 
from  which  all  our  measurements  for  the 
longitude  of  other  places  are  made ;  hence  the 
mean  time  of  all  places  to  the  eastward  of 
Greenwich  is  ahead  of  Greenwich  mean  time 
(or  G.M.T.  as  it  is  usually  called),  and  the 
mean  time  of  all  places  to  the  westward  of 
Greenwich  is  behind  G.M.T. 

As  the  revolution  of  the  earth  from  noon 
to  noon,  at  any  place,  occupies  twenty-four 
hours  for  an  angular  value  of  the  circumfer- 
ence of  a  circle,  or,  in  other  words,  360°,  it 
follows  that  longitude  may  also  be  expressed 
in  time. 

Example  : 

Arc.  Time. 

h.      m.      s. 

360°       .          .         .          .          .     24  oo  oo 
180°       .         .         .         .         .     12  oo  oo 

vO 


90°   .    .       .  6  oo  oo 

15°   .    .    .    .  i  oo  oo 

i°   .    .    .    ..  o  04  oo 
118 


EFFECT  OF  LONGITUDE~ON  TIME 

As  the  meridian  of  every  place  is  different, 
local  time  must  differ  at  any  place  from 
any  other  place,  and  would  cause  endless 
trouble  as  regards  setting  clocks. 

Consequently,  different  countries  adopt 
what  are  known  as  '  Standard  Meridians/ 
and  all  clocks  in  that  country  are  set  to 
the  time  of  that  standard  meridian. 

In  the  United  Kingdom,  except  Ireland, 
the  standard  meridian  is  that  of  Greenwich 
Observatory,  as  mentioned  before,  and  all 
clocks  are  kept  set  to  it.  N.B. — Ireland 
now  keeps  G.M.T. 

N.B. — Since  writing  this,  summer  time 
has  been  introduced  by  Act  of  Parliament. 
By  this,  clocks  are  put  on  one  hour  on  May  I 
at  midnight,  and  are  put  back  at  midnight 
on  September  30. 

An  easy  rule  to  remember  how  to  apply 
longitude  in  time  is  given  in  the  old  rhyme  : 

Longitude  west,  Greenwich  time  best, 
Longitude  east,  Greenwich  time  least. 

In  connection  with  time,  it  is  interesting 
to  understand  what  happens  to  the  day  and 
date  when  crossing  the  iSoth  meridian. 

This  is  explained  below. 

On  leaving  the  prime  meridian,  and 
119 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

steering  east,  the  ship's  local  time  gradually 
gets  ahead  of  Greenwich  time,  until  in  180° 
she  is  just  twelve  hours  in  front. 

Continuing  to  the  eastward,  she  imme- 
diately enters  a  longitude  which  is  twelve 
hours  behind  Greenwich,  so  that  she  must 
count  that  day  and  date  over  again. 

For  instance,  supposing  she  crossed  the 
iSoth  meridian  at  9  P.M.  on  August  14,  going 
east. 

When  she  got  to  the  iSoth  meridian  it 
would  only  be  9  A.M.  en  August  14  at  Green- 
wich, and,  continuing  her  course,  she  would 
at  once  be  twelve  hours  more  behind  Green- 
wich, i.e.  9  P.M.  on  August  13. 

Hence  her  next  day  and  date  must  again 
be  reckoned  as  August  14. 

On  the  other  hand,  suppose  she  sailed  from 
the  meridian  of  Greenwich,  going  west  about. 

She  would  gradually  get  more  and  more 
behind  Greenwich  time,  until  at  the  iSoth 
meridian  she  would  be  twelve  hours  late. 
On  crossing  to  the  westward  of  this  meridian, 
she  would  at  cnce  get  twelve  hours  ahead  of 
Greenwich  time,  therefore  she  must  skip  a 
day  altogether. 

For  instance,  supposing  she  crossed  the 
iSoth  meridian,  going  west,  at  9  P.M.  August  14. 

120 


HOUR   ANGLES 

When  she  was  there,  it  would  be  9  A.M. 
August  15  at  Greenwich,  and,  on  going 
farther  west,  she  would  be  twelve  hours  ahead 
of  this  latter  date,  so  that  it  would  be  9  P.M. 
on  August  15. 

Hence  she  must  skip  the  i5th  altogether, 
and  call  the  next  day  the  i6th. 

Hour  Angles. — By  the  term  '  Hour  Angle/ 
is  meant  the  angular  distance  of  a  body  from 
the  observer's  meridian  expressed  in  time, 
either  before  or  after  its  meridian  passage. 

By  '  Meridian  Passage  '  is  meant  the  cross- 
ing of  the  body  over  the  meridian  of  the 
observer. 

All  heavenly  bodies  rise  to  the  eastward 
of  the  observer,  and  after  a  certain  time  attain 
their  greatest  altitude  above  the  horizon — this 
occurs  when  the  body  is  on  the  observer's 
meridian ;  they  then  decline  in  altitude,  and 
finally  set  in  the  westward. 

This  meridian  passage  is  known  as  the 
'  Upper  Meriaian  Passage/ 

Their  lower  meridian  passage  takes  place 
twelve  hours  later  in  the  case  of  the  sun  ; 
slightly  under  (3™  56" )  twelve  hours  in" ,  the 
case  of  a  star  ;  and  an  average  of  I2h  24™  in 
the  case  of  the  moon. 

121 


AIR  NAVIGATION   FOR   FLIGHT   OFFICERS 

When  we  talk  of  a  body  being  so  many 
hours  away  from  the  meridian,  this  does  not 
mean  any  A.M.  or  P.M.  time  :  it  is  simply  a 
measure  of  time  from  its  meridian  passage. 
If  we  want  to  know  the  local  time  when  a  body 
is,  say,  three  hours  from  its  meridian  passage, 
we  must  find  out  the  sun  time  of  the  body 
crossing  the  meridian  and  apply  these  three 
hours  to  this  latter  time. 

The  reason  for  this  is  because  the  meridian 
passage  of  a  body  is  reckoned  by  mean  sun 
time,  and  so,  to  get  the  time  of  rising  or  setting 
of  any  body  other  than  the  sun,  we  must  first 
find  the  sun  time  of  the  body's  meridian 
passage  and  then  apply  its  hour  angle  from 
the  meridian  when  on  the  horizon. 

This  hour  angle  must,  of  course,  be  sub- 
tracted from  the  time  of  meridian  passage  for 
rising,  because  it  must  rise  before  it  comes  to 
the  meridian,  and  be  added  to  the  time  of 
meridian  passage  for  setting,  as  it  sets  after 
crossing  the  meridian. 

With  reference  to  the  times  shown  in  the 
sun's  true  bearing  tables  of  sunrise  and 
sunset,  it  must  be  remembered  that  as  we 
count  our  civil  day  as  beginning  at  midnight, 
so  the  actual  A.M.  time  of  sunrise,  as  given 
in  the  tables,  is  counted  from  the  inferior 

122 


EXPLANATION  OF  NAUTICAL  TABLES 

meridian  ;  so  that,  to  get  the  actual  hour 
angle  of  the  sun  from  the  superior  meridian,  we 
must  subtract  the  A.M.  time  from  twelve  hours. 
This  is,  of  course,  not  necessary  in  the 
P.M.  time,  as  our  afternoon  time  is  measured 
from  the  superior  meridian.  The  sun's  hour 
angle,  both  from  the  inferior  meridian  to  the 
superior  meridian  (A.M.  time),  and  from  the 
superior  meridian  to  the  inferior  meridian 
(P.M.  time)  is,  of  course,  apparent  time. 

Explanation   of   the   Various   Tables. — (i) 

Nautical  Almanac. — This  is  a  work  published, 
giving  all  the  data  necessary  for  navigation 
by  the  sun,  moon,  planets,  and  stars.  These 
data  are  given  for  every  day  in  the  year.  It  is 
published  in  two  forms— an  extended  form, 
and  an  abridged  form  for  the  use  of  seamen. 

The  latter  should  be  used  in  all  problems 
of  rising  and  setting,  and  is  also  sufficient  for 
all  problems  in  navigation. 

On  the  first  two  pages  of  every  month  are 
given  all  the  data  for  the  sun  and  moon. 

Each  column  indicates  the  contents  of 
that  column,  so  that  there  should  be  no 
difficulty  in  taking  out  what  is  wanted. 

The  only  column  that  needs  any  explana- 
tion is  the  one  headed  'Equation  of  Time/ 

123 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 

Here  it  gives,  at  the  top  of  the  page, 
instructions  as  to  whether  the  equation  of 
time  is  to  be  added  to,  or  subtracted  from, 
apparent  time.  In  either  case,  if  the  equa- 
tion of  time  is  to  be  applied  to  mean  time, 
the  instructions  must  be  reversed— e.g.,  sup- 
pose the  instructions  say  the  equation  of  time 
is  to  be  added  to  apparent  time,  then  it 
must  be  subtracted  from  mean  time. 

Again,  it  sometimes  happens  that  there 
is  a  black  line  drawn  both  in  the  instructions 
and  in  the  column  giving  the  values. 

This  simply  means  that  all  the  values  in  the 
column  above  the  black  line  follow  the  instruc- 
tions above  the  upper  black  line,  and  those 
below  follow  the  instructions  below  the  upper 
black  line. 

The  next  few  pages  give  the  data  for  the 
sun  for  every  day  of  the  month  at  two-hour 
intervals  of  Greenwich  mean  time  (G.M.T.). 

After  that  comes  the  same  thing  for  the 
moon,  and  these  must  be  made  use  of  when 
getting  moonrise  or  moonset.  In  the  case  of 
finding  sunrise  or  sunset,  it  will  be  near  enough 
to  take  the  declination  out  for  noon  at  Green- 
wich, as  it  does  not  alter  enough  in  twenty- 
four  hours  to  have  any  practical  effect  on  the 
accuracy  of  the  problem  ;  but  in  the  case  of 

124 


EXPLANATION    OF  NAUTICAL  TABLES 

the  moon,  her  declination  changes  so  rapidly, 
that  a  few  hours  may  make  an  appreciable 
difference  in  the  result. 

On  p.  160  of  the  Abridged  Nautical 
Almanac  will  be  found  a  table  giving  the 
hour  angle  of  a  body  from  the  meridian  when 
rising  or  setting. 

Running  right  across  the  top  of  the  two 
pages  are  the  degrees  of  declination  from 
o°  to  30°. 

Down  the  left  hand  side  of  each  page,  and 
printed  in  thick  type,  are  the  degrees  of  lati- 
tude from  o°  to  60°.  In  the  body  of  the  table 
are  the  hour  angles. 

To  look  out  an  hour  angle,  all  that  has 
to  be  done  is  to  enter  the  table  with  the 
latitude  of  the  place  and  the  declination  of 
the  body. 

Under  the  latter,  and  opposite  the  former, 
will  be  found  the  required  hour  angle. 

The  following  rule  is  important  and 
should  be  well  learnt : 

'  If  the  latitude  and  declination  are  the 
same  names,  i.e.  both  north  or  both  south, 
the  hour  angle  can  be  taken  straight  from  the 
tables  ;  but  if  they  are  different  names,  i.e.  one 
north  and  the  other  south,  the  hour  angle 
found  in  the  tables  must  be  subtracted 

125 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 

from  twelve  hours,  and  the  result  used 
instead/ 

On  p.  170  is  a  table  of  proportional  parts 
which  must  be  applied  to  the  hour  angle  as  a 
final  correction.  Across  the  top  of  the  page 
are  certain  numbers,  in  the  case  of  the  moon 
these  correspond  with  the  daily  difference  as 
given  in  the  column  after  the  upper  meridian 
passage.  Running  down  the  right  hand  side 
of  each  page  are  times  ranging  to  twelve  hours. 

The  final  correction  is  simply  a  rule-of- 
three  sum,  which  is  given  in  this  table. 

'  If  the  difference  in  twenty-four  hours  is 
so  much,  what  will  it  be  for  an  hour  angle  of 
so  much  ? '  This  hour  angle  being  the  one 
just  found. 

This  final  correction  is  always  additive 
to  the  hour  angle,  as  the  moon  crosses  the 
meridian  of  any  place  later  every  day. 

(2)  Inman's  Tables. — On  p.  116  will  be 
found  a  table  for  the  correction  of  the  moon's 
meridian  passage,  depending  on  the  ]ongitude. 
The  rule  for  adding  or  subtracting  it  is  given 
on  the  top  of  the  page. 

Just  below  it,  and  running  right  across  the 
page,  is  a  row  of  thick  figures,  which  represent 
the  daily  difference  of  the  moon's  meridian 
passage  which  is  given  in  the  Nautical 

126 


EXPLANATION  OF  NAUTICAL  TABLES 

Almanac  in  the  column  next  to  the  moon's 
upper  meridian  passage. 

Running  down  either  side  of  the  page  is  a 
column  showing  the  longitude  of  the  place, 
and  in  the  body  of  the  table  is  the  correc- 
tion to  be  applied.  This  correction  is  given 
in  minutes  of  time. 

This  table  is  merely  a  worked  out  rule-of- 
three  sum. 

'If  the  daily  difference  for  360°  is  that 
given  in  the  Nautical  Almanac,  what  is  it  for 
the  longitude  of  the  place  ?  '  Instructions 
whether  to  add  or  subtract  it  are  given  at 
the  top  of  the  table. 

Haversine  Table. — This  table  is  of  great 
use  in  giving  the  longitude  in  time  any- 
where. All  that  has  to  be  done  is  to  look 
up  the  longitude  and  take  out  the  correspond- 
ing time  shown  at  the  top  of  the  page  and 
also  down  the  sides. 

Sun's  True  Bearing  or  Azimuth  Tables 
(Davis  and  Burdwocd). — These  are  printed 
for  a  limit  -of  latitude  of  60°  north  and  south, 
and  a  limit  of  declination  of  23°  north  and 
south,  this  latter  being  approximately  the 
farthest  limits  of  the  sun's  apparent  motion 
north  or  south. 

The  times  shown  are,  as  the  sun  itself 
127 


AIR   NAVIGATION   FOR   FLIGHT  OFFICERS 

is  actually  observed,  apparent  time :  the 
intervals  as  given  in  the  tables  are  four 
minute  ones. 

It  should  be  noticed  that  the  A.M.  times 
run  up  the  left  hand  side  of  each  page,  and  the 
P.M.  times  run  down  each  page  on  the  right 
hand  side. 

With  reference  to  the  A.M.  time  of  rising, 
it  should  be  remembered  that  the  time  given 
for  rising  is  counted  from  the  inferior  or  mid- 
night meridian,  and  therefore,  to  get  the  hour 
angle  from  the  noon  or  superior  meridian,  the 
value  given  in  the  tables  must  be  subtracted 
from  twelve  hours.  This  is  not  necessary  for 
the  P.M.  hour  angle,  as  P.M.  is  counted  from 
the  time  that  the  sun  crosses  the  superior 
meridian. 

All  the  bearings  given  in  the  tables  are 
for  the  sun's  centre. 

In  both  sun  and  star  tables,  the  rules  for 
naming  the  bearing  are  the  same  in  principle, 
as  the  statement  '  When  apparent  time  is  A.M.' 
means  exactly  the  same  thing  as  '  When  the 
body  is  rising  or  east  of  the  meridian/  and 
similarly  for  P.M. 

In  the  sun  tables,  each  degree  of  latitude 
appears  over  two  separate  headings,  one 
when  latitude  and  declination  are  the  same 

128 


SUNRISE  PROBLEM 

name,  i.e.  both  north  or  both  south,  and  the 
other  when  they  are  opposite  names,  i.e.  one 
north  and  the  other  south.  Care  should  be 
taken  not  to  confuse  the  two. 

At  the  end  of  every  degree  of  declination 
is  given  the  apparent  time  of  rising  and 
setting,  and  the  true  bearing  of  the  body. 

The  star  tables  (Davis')  are  the  same  in 
principle  as  the  sun  tables,  except  that  instead 
of  the  apparent  time  being  given,  the  hour 
angle  of  the  star  is  shown. 

We  now  come  to  the  examples  of  sunrise 
and  sunset,  and  moonrise  and  moonset, 
which  are  appended.  In  practice  it  is  not 
necessary  to  work  rigorously,  so  the  declina- 
tion and  equation  of  time  may  be  taken  out 
at  sight.1  The  elements,  if  taken  out  exactly, 
only  add  to  the  time  in  working  out  without 
any  compensating  advantages,  and  make  no 
practical  difference  to  the  answer. 

Sunrise  Problem 

Example  i. — Find  the  Greenwich  mean  time  of 
sunrise  and  sunset  and  the  true  bearing  at  each 
time,  in  Latitude  50°  N.,  Longitude  8°  E.,  on 
May  7,  1916. 

From   Abridged  Nautical  Almanac,  p.   50,   on 

1  In  the  case  of  the  sun  to  the  nearest  noon,  but  with  the 
moon  to  the  nearest  hour. 

129  K 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 


May  7,  we  get  :    '  Sun's  declination    i6f  °   north. 
Same  name  as  latitude/ 

P.  207,  pt.  ii.  of  sun's  true  bearing 
tables,  with  Lat.  50°  N.  (same  name  as 
declination),  under  the  columns  headed  16 
and  17,  we  get  by  interpolation  as  follows  : 


Rising. 


h.    m.    s. 


Apparent    time    of 

rising  at  place      .  4  36  oo 
Equation    of    time, 

p.  50,  Naut.  Aim. .    — 3  30 


Mean  time  of  rising 

at  place      .          .  4  32  30 
Longitude  in  time  .  o  32  ooE- 


G.M.T.  rising      A.M.  4  oo  30 

Bearing. 
From  p.  207, 
sun's  true  bear- 
ing tables,  for 
Lat.5o°N.,Dec. 
16}°  N.  we  get 
by  interpolation  63°  23'  oo" 


Setting. 


h.     m.    s. 


Apparent    time    of 

setting  at  place.   7  24  oo 
Equation    of    time, 

p.  50,  Naut.  Aim.     — 3  30 


Mean    time  of    set- 
ting at  place  .        7  20  30 
Longitude  in  time  .  o  32  ooE. 


G.M.T.  setting  P.M.  6     8  30 

Bearing. 
From  p.  207, 
sun's  true  bear- 
ing tables,  for 
Lat.50°N.,Dec. 
i6|°N.weget 


163°  23'  oo" 
360°  oo7  oo' 
296°  37'  oo' 


Therefore  the  answer  to  the  problem  is 
that  the 

Sun   rises   at   4h    oom    30"    A.M.,    bearing 
63°  23'  o". 

130 


SUNRISE  PROBLEM 


Sun  sets  at  6h  48™  3OS  P.M.,  bearing 
296°  37'  oo". 

These  of  course  are  true  bearings ;  should 
magnetic  bearings  be  required,  the  variation 
must  be  applied. 

If  local  mean  time  be  required,  of  course 
the  longitude  in  time  would  not  be  applied. 

Example  2.- — Find  G.M.T.  of  sunrise  and  sunset 
and  the  true  bearing  at  each  time  in  Latitude  40°  S., 
Longitude  10°  W.,  on  August  5,  1916. 

From  Nautical  Almanac  :  '  Sun's  declination  is 
17°  north,  i.e.  contrary  name  to  latitude/ 


h.    m.    s. 


Rising. 

Apparent    time    at 

place         .          .       6  56  oo 
Equation  of  time     -|~  °  °^  °° 


Mean  time  at  place       7  02  oo 


Long,  in  time 
G.M.T.  rising 


W-j-  o  40  oo 


h.    m.    s. 


Setting. 

Apparent    time    at 

place         .          .       5  04  oo 
Equation  of  time   .  -f-  o  06  oo 


Mean  time  at  place       5  10  oo 
Long,  in  time       W-f-  o  40  oo 


A.M.  7  42  oo     G.M.T.  setting       P.M.  5  50  oo 


Bearing. 

S.  in0  05'  E. 

i.e.    68°  55'  from    rule    given 
before. 


Bearing. 

S.  in0  05'  W. 

i.e.   291°  05'  from  rule  given 
bef  ore. 


Examples  on  Moonrise  Problem 
Example  I. — Find  the  time  of  moonrise  and 


AIR   NAVIGATION   FOR   FLIGHT   OFFICERS 


moonset  and  the  moon's  true  bearing  at  each  in 
Latitude  40°  N.,  Longitude  70°  W.,  on  June  5, 1916. 


From  Naut.  Aim., 
p.  63,  moon's  mer. 
pass  at  upper 
transit  on  June  5 

Correction    in     In- 
man's          tables, 


h.    m.    s. 

3  43  oo 


p.  116 


. -f-  o  09  oo 


Corrected      local 

mean      time      of 

passage      .          .       3  52  oo 
Longitude  in  time  .  -{-  4  40  oo 


Rough    G.M.T.    of 

passage,  June  5   .       8  32  oo 
Moon's  hour  angle         6  25  oo 

Rough    G.M.T.     of 

rising,  June  5       .       2  07  oo 
Rough     G.M.T.     of 

setting,  June  5   .       14  57  oo 


Daily  difference  in 
next  column  to 
upper  transit 


44r 


Moon's  declination  for 
8h  32™  G.M.T.  on 
June  5  .  .  7*°  N. 

Same  name  as  Latitude. 

From  p.  1 60,  Naut.  Aim., 
with  Lat.  40°  N.,  Dec.  7^°  N. 
Moon's  hour  angle  is  6h  25™  oos 

This  means  that  the  moon, 
when  rising  and  setting,  is 
6h  25™  away  from  the  time  of 
her  passage  over  the  observer's 
meridian  as  shown  by  sun  mean 
time: 


Owing  to  the  rapid  change  in  the  moon's 
declination,  the  example  must  now  be  re- 
worked, using  the  rough  G.M.T.  times  of  rising 
and  setting  to  get  the  declinations. 

Rising. 


Moon's    dec.    for  2h  07™, 
G.M.T.,  Junes         .     18°  N. 

Both  same  name  as  latitude. 
132 


Setting. 
Moon's  dec.  for  i4b  57™, 

G.M.T.,  June  5      .     i5f°  N. 


MOONRISE  PROBLEM 


Rising. 
Hour     angle     from 

p.i6o,Naut.Alm., 

with  Lat.  40°  N.,      h.   m.     s. 

Dec.  18°  N.  7  03  oo 

Correction   p.    170, 

Naut.  Aim.,  with 

daily     difference 

44m     and      hour 

angle  yh  03™  -f-  13  oo 

Corrected          hour 

angle  .          .       7  16  oo 

Corrected          local 

mean     time     of 

passage,  June  4   .     27  52  oo 


Moon  rises  June  4    .     20  36  oo 
I.e.moon  rises  June5, 

civil  time  .       A.M.    8  36  oo 


h.    m.    s. 

6  55  oo 


Setting. 
Hour     angle     from 

p.i6o,Naut.Alm., 

with  Lat.  40°  N., 

Dec.  I5|°  N. 
Correction   p.    170, 

Naut.  Aim.,  with 

daily      difference 

44m      and      hour 

angle  6h  55™ 

Corrected          hour 

angle  .          .       7  08  oo 

Corrected          local 

mean     time     of 

passage,  June  5  .       3  52  oo 


+  13 


Moon  sets  June  5     .     1 1  oo  oo 
Moon  sets  June  5, 

civil  time          P.M.  n  oo  oo 


Moon's  Bearing. 
With  Lat.  40°  N., 
Dec.  i8°N.,  same 
name  as  lat. 
true  bearing  from 
tables  .  .  66°  13'  oo' 


Moon's  Bearing. 
WithLat.4o°N., 

Dec.    I5f°    N.,         69°  15'  oo" 
same  name  as       360°  oo'  oo" 

lat.  true  bear-     

ing  from  tables       290°  45'  oo" 


Note. — In  the  second  part  of  the  foregoing 
problem,  it  should  be  noticed  that  under  the  rising 
heading  the  corrected  local  mean  time  of  passage 

133 


AIR  NAVIGATION   FOR   FLIGHT  OFFICERS 


has  been  given  as  27*  52™  oos.  This  simply  means 
that  24h  has  been  added  on  to  the  original 
3h  52m  oos.,  as  7h  i6m  oos  has  to  be  subtracted 
from  it.  By  adding  24h  to  it,  the  date  has,  of 
course,  to  be  placed  one  day  back. 

Example  2. — Find  the  time  of  moonrise  and 
moonset  and  the  true  bearing  at  each  time  in 
Latitude  50°  S.,  Longitude  100°  E.,  on  October  12, 
1916. 


From   Naut.    Aim., 

p. in, moon's  mer. 

passage  at  upper     h.     m.   s. 

transit  on  Oct.  12     13  04  oo 
Correction    in     In- 

man'stables,p.n6 — oo  14  oo 


Corrected          local 

mean     time      of 

passage,  Oct.  12  .      12  50  oo 
Longitude  in  time  .  —  6  40  oo 

Rough    G.M.T.    of 

passage,  Oct.  12  .       6  10  oo 
Moon's  hour  angle .       4  26  oo 


Rough    G.M.T    of 

rising,  Oct.  12     .        i  44  oo 
Rough    G.M.T.    of 

setting,  Oct.  12    .     10  36  oo 


Daily  difference  in 
next  column  to 
upper  mer.  pass  .  52 


Moon's      declination 
for    G.    M.    T.    of 
6h  iom  oos  Oct.  12     i8|°  N- 
Opposite  name  to  latitude. 

From  p.  160,  Naut.  Aim., 
with  Lat.  50°  S.,  Dec.  i8£°  N. 
Hour  angle  is  7h  34™. 

Subtract  this  from  I2h, 
because  lat.  and  dec.  are 
opposite  names. 

Moon's  hour  angle  from 
meridian  is  therefore  4h  24™  oos. 


Re-working  the  problem  as  in  Example  I, 
we  get  as  follows  : 


MOONRISE  PROBLEM 


Rising. 
Moon's  dec.  for  ih  44™, 

G.M.T.  Oct.  12  1 8°  N. 


Setting. 

Moon's  dec.  for  ioh  36* 
G.M.T.  Oct.  12 


Both  opposite  name  to  latitude. 


Hour     angle     from 
p.i6o,  Naut.Alm., 


Hour     angle     from 
p.  1 60,  Naut.Alm., 


with  Lat.  30°  E.,      b.    m.   s. 
Dec.  i8°N.           .       7  31  oo 
Subtract  from  I2h  .      12  oo  oo 

with  Lat.  50°  S.,       h.   m.  s. 
Dec.  ig£°  N.       .       7  40  oo 
Subtract  from  i2h  .     12  oo  oo 

Moon's  hour  angle  .       4  29  oo 
Correction    p.    170, 
Naut.  Aim.,  with 
daily     difference 
52,  and  hour  angle 
4h  29™  oos  .        .      -f-  10  oo 

Moon's  hour  angle  .       4  20  oo 
Correction    p.    170, 
Naut.  Aim.,  with 
daily      difference 
52,  and  hour  angle 

4h   2Om   00s    .           .        -|~    IO   °° 

Corrected           hour 
angle  of  moon     .       4  39  oo 
Local  mean  time  of 
passage,  Oct.  12  .      12  50  oo 

Corrected       hour 
angle  of  moon     .       4  30  oo 
Local  mean  time  of 
passage,  Oct.  12.      12  50  oo 

Moon  rises  Oct.  12.       8  n  oo 
I.e.       civil        time 
Oct.  12        .       P.M.    8  ii  oo 

Moon's  Bearing. 
With    Lat.  50°    S., 
Dec.   1  8°  N.,   op- 
posite   name     to 
lat.  bearing  from 
tables           .          .      1  1  8°  44' 
Subtract  fro  mi  80°  .     180°  oo' 

Moon  sets  Oct.  12  .     17  20  oo 
I.e.       civil       time 
Oct.  13       .       A.M.    5  20  oo 

Moon's  Bearing. 
With  Lat.,    50°    S., 
Dec.  iQi0  N.,  op- 
posite   name     to 
lat.  bearing  from 
tables          .          .       121°  17' 
Add  180*       .          .       1  80°  oo' 

Moon's  true  bearing         61°  16' 

Moon's  true  bearing      301°  17' 

135 


AIR  NAVIGATION   FOR  FLIGHT   OFFICERS 

It  cannot  be  too  often  stated  that  if  the 
latitude  and  decimation  are  of  opposite  names, 
the  hour  angle  found  on  pp.  160-1  of  the 
Abridged  Nautical  Almanac  must  be  sub- 
tracted from  twelve  hours,  and  the  result 
substituted. 

The  declination  of  the  moon  may  go  up  as 
high  as  29°  on  either  side  of  the  equator,  so 
that  after  23°  the  sun  tables  are  not  available  ; 
in  this  case  the  star  tables  may  be  used,  using 
vols.  i  or  2  according  to  the  latitude. 

The  principle  of  looking  out  the  bearings 
is  exactly  the  same  as  in  the  sun  tables,  or 
the  amplitude  tables  in  Inman's  may  be  used. 

In  the  left-hand  column  of  the  star  tables 
will  be  found  the  body's  hour  angle — that 
is,  its  angular  distance  from  the  meridian 
ex  ressed  in  time. 

Opposite  the  hour  angle  and  under  the 
declination  of  the  body,  will  be  found  the 
true  bearing. 

These  bearings  are,  however,  only  given 
for  when  the  body  is  some  degrees  above  the 
horizon,  consequently  interpolation  will  be 
necessary. 

As  the  rate  of  change  of  the  bearing  of 
a  body  varies  with  its  altitude,  declination, 
and  position  of  the  observer,  this  interpola- 

136 


STAR  TABLES 

tion  will  not  be  rigorously  exact,   but  for 
compass  work  it  will  be  quite  near  enough. 

The  rule  for  naming  the  bearing  is,  like 
in  the  sun  tables,  given  at  the  foot  of  each 
page. 

Example.— Lai.  40°  N.,  Dec.  30°  N.  Required 
true  bearing  at  rising  and  setting. 

By  table  on  pp.  160-1  of  Nautical  Almanac 
the  body's  hour  angle  when  rising  and  setting  is 
7h  56m  oos. 

On  p.  33  of  Davis'  star  tables,  for  40° 
lat.  and  under  30°  dec.,  same  name,  the  first 
bearing  given  after  the  body  is  above  the 
horizon,  is  for  an  hour  angle  of  7h  5om  os,  and 
is  5o0<2. 

Now  the  change  between  the  bearing  for 
7h  40m  and  7h  50™  is  i°'6,  and  bearing 
increasing. 

Therefore  the  change  for  six  minutes  is 
A  of  i°-6. 

That  is  to  say,  the  change  is  -96  of  a  degree, 
say  i  degree.  Therefore  the  bearing  at  rising 
and  setting  will  be  5o°'2  — 1°'0.,  i.e.  49°*2, 
named  according  to  the  rule  at  the  foot  of  the 
page.  So  that  bearing  at  rising  will  be  49°*2, 
and  at  setting  36o°*oo  —  49°*2.,  i.e.  3io°'8. 

As  stated  before,  these  bearings  are  not 


AIR   NAVIGATION   FOR  FLIGHT    OFFICERS 

absolutely  exact,  but  are  near  enough  for 
compass  work. 

In  connection  with  these  bearings,  there 
is  another  method  of  looking  out  the  true 
bearings  at  rising  and  setting.  This  can  be 
done  by  means  of  the  '  Amplitude  Table ' 
given  on  pp.  138-41  of  Inman's  tables. 

Before  explaining  the  tables,  it  may  be 
as  well  to  state  that  an  amplitude  is  merely 
the  bearing  of  the  body  when  rising  or  setting, 
reckoned  from  the  east  or  west  point  according 
as  to  whether  the  body  is  rising  or  setting. 

It  differs  from  an  azimuth,  inasmuch  as 
the  latter  is  reckoned  from  the  north  point,  and 
the  amplitude  only  applies  to  a  body  when  on 
the  horizon. 

Running  across  the  top  of  the  pages  are 
the  degrees  of  declination,  and  down  the  sides 
are  the  degrees  of  latitude  from  i  to  64. 

Under  each  degree  of  declination  are  two 
columns,  one  headed  '  Time  Amp/  and  the 
other  '  Bearing  Amp/ 

The  time  amplitude  is  merely  an  interval 
of  time  to  be  added  to  or  subtracted  from 
6h  oom  oo3,  which  will  give  the  hour  angle  of 
the  body  from  the  superior  meridian  expressed 
in  time. 

Whether  it  should  be  added  or  subtracted 
from  6h  oom  oos  depends  on  whether  the 

138 


AMPLITUDES 

latitude  and  declination  are  of  the  same  or 
opposite  names. 

If  they  are  of  the  same  name,  the  hour 
angle  of  rising  must  be  greater  than  6h  oom  oos ; 
and  if  they  are  of  opposite  names,  the  hour 
angle  of  rising  must  be  less  than  6h  oom  oos. 

Similarly  in  the  case  of  setting. 

Therefore,  if  the  latitude  and  declination 
are  of  the  same  name,  the  time  amplitude 
found  in  the  tables  must  be  added  to  6h  oom  oos ; 
and  if  they  are  of  different  names,  it  must  be 
subtracted  from  6h  oom  oos. 

With  regard  to  the  bearing  amplitude,  if 
latitude  and  declination  are  of  the  same  name, 
the  body  must  rise  north  of  the  east  and  west 
line,  and  also  set  north  of  it.  If  they  are  of 
opposite  names,  the  body  must  rise  south 
of  the  east  and  west  line  and  set  south  of 
it.  This  will  at  once  show  which  way  the 
bearing  amplitude  should  be  applied  to  the 
east  or  west  point. 

This  paragraph  refers  to  north  latitude; 
for  south  latitude,  if  latitude  and  declination 
are  the  same  names,  the  body  will  rise 
south  of  the  east  and  west  line  and  also 
set  south  of  it,  whilst  if  latitude  and  declina- 
tion are  of  opposite  names,  the  body  will 
rise  north  of  the  east  and  west  point  and  set 
north  of  it. 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 

Taking  the  example  given  for  the  star 
tables,  it  is  proposed  to  work  it  out  by  the 
amplitude  table  as  well. 

Example.- — Latitude  40°  N.,  Declination  30°  N. 
Find  hour  angle  of  body  when  rising  and  setting, 
also  true  bearing  at  each  time. 

h.    m.     s. 

P.  141,  Inman's  tables,  under  30°  and  opposite 

40°,  time  amplitude  is          .          .          .          .        i  56  oo 
Lat.  and  dec.  being  same  names,  add  6h  oom  oos-f  6  oo  oo 


Time  amp.  or  hour  angle  of  body  from  superior 

meridian  on  rising 7  56  oo 

Which  agrees  with   the  hour  angle    given 

in  the  last  example. 
P.  141,  Inman's  tables,  under  30°  and  opposite 

40°,  bearing  amp.  is     .          .          .          .          .         40°  -8 
[As  lat.  and  dec.  are  same  names,   this 

will  be  north  of  the  east  point.] 
East  point   ......  .         90°  -o 

Bearing  of  body 49°  -2 

Which  agrees  with  bearing  given  in  the  last 
example. 

Similarly,  the  setting  hour  angle  will  be 
6h  oom  oos  +  ih  56™  oos,  which  gives  7h  56™  oo 
from  the  meridian.  And  the  bearing  will  be 
4O°'8  north  of  the  west  point,  which  gives  a 
bearing  of  270°  oo'  oo"  +  40°*8  or  3io°*8. 
Or  9o0<o  —  40*8  =  49-2,  west  of  the  north  point 

140 


AMPLITUDES 

and  36o°-o  —  49°'2  =  3io°'8.  This  agrees  with 
the  bearing  given  in  the  first  example. 

The  east  and  west  points  are  reckoned  as 
being  6h  oom  oos  in  time,  and  90°  in  arc  away 
from  the  north  and  south  points. 

The  following  figures  may  be  of  assistance 
for  amplitude  work. 


FIG.  61.— North  Lat.  Lat. 
and  Dec.  same  name. 

Body  rises  at  X  and 
sets  at;  X',  XZE  and 
X'ZW  are  the  ampli- 
tudes from  the  tables, 
and  X  and  X7  must  be 
north  of  the  east  and 
west  line,  so  that  the 
hour  angles  SZX  and 
SZK'  are  greater  than 
90°,  i.e.  greater  than 
6hoom  oos,  so  that  amps, 
must  be  added. 


FIG.  62.— North  Lat.  Lat 
and  Dec.  opposite  names. 

Body  rises  at  X  and 
sets  at  X',  XZE  and 
X'ZW  are  the  ampli- 
tudes from  the  tables, 
and  X  and  X'  must  be 
south  of  the  east  and 
west  line,  so  that  the 
hour  angles  SZX  and 
SZX'  are  less  than  90°, 
i.e.  less  than  6h  oom  oos, 
so  that  amps,  must  be 
subtracted. 


141 


AIR    NAVIGATION   FOR   FLIGHT   OFFICERS 

N 


FIG.  63.— South  Lat.    Lat. 
and  Dec.  same  name. 

Body  rises  at  X  and 
sets  at  X'.  Same  re- 
marks as  in  Fig.  61  apply. 


FIG.  64.— South  Lat.    Lat. 
and  Dec.  opposite  names. 

Body  rises  at  X  and 
sets  at  X'.  Same  re- 
marks as  in  Fig.  62  apply. 


142 


CHAPTER  X 
CHART  WORK 

Admiralty  Charts. — These  are  of  two  kinds — 
the  '  Gnomonic  '  and  the  '  Mercator's.' 

The  gnomonic  projection  is  used  for  plans 
of  harbours,  where  the  scale  of  the  chart 
exceeds  two  inches  to  the  mile,  for  charts 
above  the  latitude  of  about  70°  north  and 
south,  and  for  polar  charts. 

It  is  constructed  on  the  following  principle  : 


AIR    NAVIGATION   FOR  FLIGHT  OFFICERS 

The  observer  is  supposed  to  be  situated 
at  C,  the  centre  of  the  earth,  which  is  sup- 
posed to  be  transparent  so  that  he  can  see 
the  surface. 

A  is  the  central  point  of  the  part  to  be 
surveyed,  and  from  this  point  a  tangent  DAB 
is  drawn  to  the  earth's  surface.  This  point 
A  is  known  as  the  '  Point  of  Tangency/ 

The  arc  GAE  of  the  earth's  surface  is  the 
part  to  be  surveyed,  and  lines  CG,  CA,  CB 
are  drawn,  produced  if  necessary,  to  cut  the 
tangent  to  the  earth's  surface  at  D,  A,  and  B 
respectively. 

Hence  the  arc  GAE  will  be  represented  by 
the  straight  line  DAB. 

Reference  to  the  figure  will  show  that  CA 
being  at  right  angles  to  the  line  DAB,  the 
observer  is  looking  directly  at  A,  and  at  any 
other  point  on  this  line  he  will  be  looking 
more  and  more  obliquely  as  D  and  B,  the 
extremities,  are  approached,  the  maximum 
being  at  the  points  D  and  B. 

Hence  at  A  there  will  be  no  distortion,  but 
this  will  increase  all  round  on  leaving  A, 
reaching  a  maximum  at  the  edges  of  the  chart. 

The  amount  of  distortion  of  the  arc  GAE 
will  be  represented  approximately  by  the 
amount  DH-BF. 

144 


H  MERCATOR'S  CHART 

The  plan  of  a  harbour,  representing  as  it 
does  such  a  very  small  portion  of  the  earth's 
surface,  has  practically  no  distortion  ;  but  in 
a  polar  chart,  embracing  as  it  does  a  big 
area,  may  have  a  considerable  amount. 

The  Mercator's  Chart. — This  principle  is 
used  for  general  charts,  coasting  sheets 
and  between  the  limits  of  about  70°  north 
and  south.  After  about  70°  the  distortion 
becomes  so  rapid  and  excessive  that  its 
use  is  prohibitive. 

The  principle  of  construction  is  as  follows  ; 


FIG.  66. 


FIG.  67. 


In   Fig.  66  imagine  a  cylinder  of  paper 
ABCD  to  be  wrapped  round  a  flexible  globe 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

marked  with  the  meridians  and  parallels,  so 
that  it  is  touching  along  every  point  round 
EQ,  the  globe's  equator. 

If  the  globe  be  now  blown  out  until  every 
point  on  its  surface  touches  the  cylinder,  and 
the  latter  be  then  removed  and  laid  out  flat, 
it  will  be  found  that  all  the  meridians  and 
parallels  are  represented  by  straight  lines  at 
right  angles  to  one  another. 

Fig.  67  gives  a  section  of  the  earth  from 
pole  to  equator. 

It  will  be  readily  seen  that  along  the  line 
EQ,  or,  in  other  words,  along  the  equator,  there 
has  been  no  distortion,  as  the  cylinder  was 
already  touching  the  globe. 

As  one  goes  towards  either  pole,  it  will  be 
seen  from  the  figure  that  the  parallel  of  lati- 
tude DE  has  been  expanded  to  the  length 
CF,  and  the  parallel  HK  to  the  length  GL. 
As  the  parallel  HK  is  less  than  DE,  and  as  GL 
and  CF  are  equal  to  one  another,  it  follows 
that  HK  must  have  been  expanded  a  greater 
amount  than  DE. 

Hence,  as  the  poles  are  approached,  the 
expansion  must  get  greater  and  greater. 

As  AB,  GL,  CF,  and  EQ  are  all  equal,  the 
degrees  of  longitude  on  a  Mercator's  chart 
must  be  represented  by  parallel  straight 

146 


MERCATOR'S  CHART 

lines,  and  therefore  they  must  be  all  equal  to 
one  another. 

The  degrees  of  longitude  having  been 
expanded  on  an  increasing  scale  as  the  poles 
are  approached,  the  proportion  of  the  chart 
must  be  preserved  by  expanding  the  degrees 
of  latitude  in  the  same  proportion  as  the 
degrees  of  longitude  have  been. 

And  as  this  expansion  becomes  greater 
as  the  latitude  is  increased,  the  degrees  of 
latitude  will  become  larger  and  larger  from 
the  equator  to  the  north  and  south. 

For  this  reason,  when  measuring  distance 
on  a  Mercator's  chart,  the  latitude  scale 
should  always  be  used,  and  if  the  two  places 
are  far  apart  in  latitude,  the  mean  of  middle 
latitude  must  be  taken  as  the  measuring 
point. 

Theoretically,  a  Mercator's  chart  can  be 
constructed  nearly  up  to  the  pole  itself,  but 
the  construction  fails  here,  because  the  pole, 
being  a  point,  has,  according  to  Euclid,  no 
parts  and  no  magnitude,  and  would  therefore 
have  to  be  expanded  to  infinity. 

In  practice,  Mercator's  charts  are  not 
constructed  for  a  higher  latitude  than  about 
70°  north  or  south,  as  after  that  the  distortion 
increases  very  rapidly,  and  the  degrees  of 

147 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 

latitude  get  so  very  long,  that  the  chart  would 
become  unwieldy  owing  to  its  size. 

A  Mercator's  chart  is  constructed  accord- 
ing to  the  following  method,  of  which  an 
example  is  now  given. 

Supposing  it  is  required  to  construct  a 
Mercator's  chart  on  a  scale  of  '  x  '  inches  to 
a  degree  of  longitude,  between  certain  limits 
of  latitude  and  longitude. 

The  only  table  required  is  the  one  in 
Inman's  tables,  called  '  Meridional  Parts/ 

This  table  merely  gives  the  distance 
represented  on  a  Mercator's  projection,  of  any 
distance  from  the  equator,  instead  of  the 
true  one. 

For  instance  : 

Latitude  50°,  50°  x  60'  =  3000',  i.e.  the 
parallel  of  50°  is  3000'  from  the  equator. 
The  table  of  meridional  parts  gives  for  latitude 
50°  3474-47  miles ;  this  means  that,  according 
to  the  Mercator's  projection,  the  parallel  of 
latitude  50°  would  be  drawn  in  3474*47  miles 
from  the  equator. 

Example. — Construct  a  Mercator's  chart 
between  the  parallels  of  50°  and  54°  north 
latitude,  and  between  the  meridians  of  3° 
and  7°  east  longitude,  on  a  scale  of  two 
inches  to  one  degree  of  longitude. 

148 


CONSTRUCTION  OF  MERCATOR'S  CHART 


The  rule  is  : 

Length  of  a  degree  of  latitude  equals 
Difference  between  its  limiting  meridian 
parts  multiplied  by  scale  of  longitude  and 
divided  by  60'. 

54° 


3"-363 

53° 

3"-256 

52° 

3"-212 

51° 

3"-145 


3°      2"      4C 


5°      2"      6' 


FIG.  68. 

Draw  in  the  lower  horizontal  line,  and 
mark  it  off  in  equal  spaces  of  two  inches  each 
to  the  limits  of  the  longitude  required. 

149 


AIR   NAVIGATION   FOR  FLIGHT  OFFICERS 


Draw  perpendiculars  to  each  of  the  ends 
of  this  line.  It  is  now  required  to  measure  off 
along  these  perpendiculars  the  length  of  each 
degree  of  latitude. 

Lat.  50°  Mer.  Parts  .          .          .          .         3474*47 
Lat.  51°  Mer.  Parts  .  .         .         3568*81 

Difference 


94'34 
X2 


60 


188*68 


3*145 

Therefore   the   length    of   the    degree    of 
latitude  between  50°  and  51°  is  3*145  inches. 


Lat.  51°  Mer.  Parts 
Lat.  52°  Mer.  Parts 
Difference 


3568-81 

3665*10 

96-38 

X2 


60 


192*76 


3'2I2 

Therefore   the   length    of   the    degree   of 
latitude  between  51°  and  52°  is  3*212  inches. 


Lat.  52°  Mer.  Parts 
Lat.  53°  Mer.  Parts 
Difference 


3665*19 

3763*76 

98-57 

X2 


6o 


150 


3-286 


CONSTRUCTION  OF  MERCATOR'S  CHART 

Therefore   the   length    of   the   degree   of 
latitude  between  52°  and  53°  is  3*2S6  inches. 

Lat.  53°  Mer.  Parts  .          .          .         3763*76 

Lat.  54°  Mer.  Parts  .          .          .         3864-64 


Difference         .  .  iou'8d 

X2 


60    20176 


3^63 

Therefore  the  length  of  this  degree  will  be 
3*363  inches. 

These  distances  can  now  be  measured  off 
along  the  perpendicular  lines  and  the  remain- 
ing necessary  meridians  and  parallels  put  in. 

One  great  advantage  of  a  Mercator's 
chart  is,  that  the  course  between  any  two 
places  can  be  found  by  joining  the  two, 
placing  a  parallel  ruler  along  this  line,  and 
transferring  it  to  one  of  the  compasses  en- 
graved on  the  chart.  Where  the  ruler  cuts 
the  graduated  circle  on  the  compass  will  be 
the  course  required. 


CHAPTER  XI 

INFORMATION  GIVEN  ON  AN  ADMIR- 
ALTY CHART,  CONVENTIONAL  SIGNS 
AND  SYMBOLS 

THE  information  given  on  an  Admiralty 
chart  is  expressed  by  means  of  certain  signs 
and  symbols,  which  should  be  carefully 
studied,  as  by  knowing  them  thoroughly  the 
various  markings  can  be  read  at  a  glance 
like  the  print  in  a  book. 

On  the  seaward  part  of  the  chart  are  given 
the  soundings  or  depth  of  water  at  a  certain 
standard  state  of  the  tide,  the  various  banks 
and  shoals  with  the  depths  over  them,  arrows 
showing  the  direction  of  the  tidal  streams, 
the  various  harbours,  lights,  light  vessels, 
buoys,  etc. 

Soundings  on  banks  which  are  underlined 
may  mean  two  things  :  either  the  amount  they 
uncover,  or  the  depth  on  them  at  high  water. 
This  can  always  be  ascertained  by  looking  at 
the  title  of  the  chart. 

On  the  land  part  of  the  chart  are  given 
152 


INFORMATION  GIVEN  ON  CHARTS 

the  general  topography  of  the  coast,  the 
nature  of  the  coast  line,  whether  rocky,  cliffy, 
sandy,  etc.  ;  the  various  lighthouses,  towns, 
harbours,  hills,  roads,  villages,  railways,  etc. 
The  topography  is,  however,  not  given  in  such 
detail  for  any  distance  inland  as  it  is  in  an 
ordnance  map,  as  it  is  not  so  much  required 
by  the  seaman. 

On  one  side  of  the  chart  is  engraved 
what  is  known  as  the  '  Title  of  the  Chart '  ; 
the  information  contained  in  this  is  im- 
portant, and  should  be  carefully  studied  for 
each  chart. 

The  date  of  printing  is  given  on  the  lower 
margin  of  the  chart. 

When  using  an  Admiralty  chart,  it  must 
be  remembered  that  the  nautical  mile  is  used 
as  a  unit,  which  is  equivalent  to  6000  feet 
in  length,  and  this  nautical  mile  is  sub- 
divided into  ten  '  cables  '  of  600  feet  each. 

In  the  Admiralty  chart  drawn  on  the 
Mercator's  principle,  the  latitude  scale  will 
be  found  running  up  and  down  the  sides  of 
the  chart,  and  the  longitude  scale  along  the 
top  and  bottom. 

This  longitude  scale  must  only  be  used 
for  measuring  the  difference  of  longitude 
between  two  places,  and  never  for  distance. 


AIR  NAVIGATION   FOR    FLIGHT    OFFICERS 

In  an  Admiralty  plan  a  scale  of  latitude 
and  distance  is  always  given,  and  usually  a 
scale  of  longitude. 

Should  the  latter  not  be  shown,  it  is  easy 
to  construct  one  if  required,  and  the  method 
of  doing  this  will  be  given  later. 

Conventional  Signs  and  Symbols  in  Use  on 
Admiralty  Charts. 


Steep  coast. 
o§>(4feeth,gh)  Islands  and  rocks. 

Cliffs. 

Sandy  shore. 

Shingle  or  stony  shore. 
Breakers. 


CHART  SYMBOLS 


Stones,  shingle,  or 
gravel,  dry  at 
L.W.O.S. 


Mud,  dry  at  L.W.O.S. 


Sand,  gravel,  or' stones, 
dry  at  L.W.O.S. 


%M*$'l>,$    Rocky     edges    which 

fev  &/     Wtis 


cover  an    uncover. 


Sandy  beach. 

Sand  banks,  dry  at 
L.W.O.S.  Figures  on 
banks  denote  either 
amount  they  un- 
cover at  L.W.O.S.  or 
depth  at  H.W.O.S. 
This  information  is 
always  given  on  title 
of  chart. 

Sand  hills 


155 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 


Trees. 


Cultivated  land. 


Towns. 


^  ^t^%S4^  Swamp  or  marsh. 


Church  or  chapel. 

Beacon,  flagstaff,  or 
chimney. 

156 


CHART  SYMBOLS 

Windmill. 

Roads": 
ist  JDlass. 

2nd  Class. 
Track. 


Railway. 


Tramway. 


Wreck 
(1910) 


Rock  awash  atL.W.O.S. 

Rock  with  less  than 
six  feet  of  water  at 
L.W.O.S 

Wreck  submerged. 


o     c> 

(3 


Rocks    with     limiting 
danger  lines. 


Kelp. 


AIR  NAVIGATION   FOR    FLIGHT   OFFICERS 


miiiiijiiiiiiiiimiiiiiiiniiiii  iiiiiiimmiinimiiiiiiiiii 


Beacons. 

Light  vessels. 

Fathom  Lines 

1  fathom. 

2  fathoms. 

3  fathoms. 


4  fathoms. 

5  fathoms. 
158 


ABBREVIATIONS  ON  CHARTS 

6  fathoms. 

so  on  till     10 
._._. I0  fathoms. 


20  fathoms. 

so  on  till     90 


JL\S\J         J.U/U 

Quality  of  the  Bottom  of 

the  Sea 

b 

Blue. 

m 

Mud. 

blk 

Black. 

mus 

Mussels. 

br 

Brown. 

oz 

Ooze. 

brk 

Broken. 

peb 

Pebbles. 

c 

Coarse. 

r 

Rock. 

chk 

Chalk. 

s 

Sand. 

cl 

Clay. 

sft 

Soft. 

crl 

Coral. 

sh 

Shells. 

d 

Dark. 

shin 

Shingle. 

f 

Fine. 

spk 

Speckled 

g 

Gravel. 

st 

Stones. 

gn 

Green. 

w 

White. 

grd 

Ground. 

wd 

Weed. 

gy 

Grey. 

y 

Yellow. 

h 

Hard. 

159 


AIR    NAVIGATION   FOR   FLIGHT   OFFICERS 

Tidal  Abbreviations 

Equin1.          Equinoctial.     L.W.O.S.      Low  Water 


Fl. 

Flood. 

Ordinary 

H.W. 

High  Water. 

Springs. 

H.W.O.S. 

High  Water 

m. 

Minute-s. 

Ordinary 

Np. 

Neaps. 

Springs. 

ord. 

Ordinary. 

h. 

Hour-s. 

Qr- 

Quarter. 

kn. 

Knot-s. 

Sp.  or  Spr. 

Springs. 

L.W. 

Low  Water. 

© 


Eddies. 


=^=^  ^^^^-  Overfalls. 


Abbreviations  for  Buoys 


B.,  Blk.  Black. 
Cheq.       Chequered. 
G.  Green. 

Gy.          Grey. 
H.S.        Horizontal 
Stripes. 

No.          Number. 
R.  Red. 


S.B.  j  Submarine  Fog 
Bell  (sounded  by 
wave  action) . 

S.F.B.  Submarine  Fog 
Bell  (mechanic- 
ally sounded). 

V.S.        Vertical  Stripes. 

W.  Wh.  White. 

Y.  Yellow. 

160 


ABBREVIATIONS  ON  CHARTS 


Abbreviations  for  Lights 

V.,  Lts.   Light-s. 

V  Alt.  *  Light  Alternat- 
ing. 

V  F.     *Light  Fixed. 

V  Fl.     *Light  Flashing. 

V  Occ.  *  Light  Occult- 
ing. 

V  Rev.  *Light  Revolv- 
ing. 

LtF.FL.*Light  Fixed  and 
Flashing. 

L*  Gp.  *Light  Group 
Fl.(2)  Flashing. 

LlF.Gp.*Light  Fixed 
Fl.(3)  and  Group 
Flashing. 

L*  Gp.   *Light       Group 

Occ.  (3)         Occulting. 

*  Position  of  lights. 

N.B. — Figures  in  parenthesis  after  the  de- 
scription of  a  light,  denote  the  number  of 
flashes  or  occultations  in  its  cycle  or  phase. 

General  Abbreviations 
B.  Bay. 

Baty.         Battery. 
Bk.,  Bks.  Bank-s. 
Bn.,  Bns.  Beacon-s 
Br.  Bridge. 


Alt. 
ev. 
fl.,  fls. 

Alternating. 
Every. 
Flash-es. 

G.,  Gn. 

Green. 

Gp. 

hor1. 

Group. 
Horizontal. 

irreg. 
m. 

Irregular. 
Miles. 

min. 

Minute-s. 

obscd. 

Obscured. 

occas1. 

Occasional. 

R. 

Red. 

sec. 

Second-s. 

(U) 

vert1. 

Unwatched. 
Vertical. 

vis. 

Visible. 

W.,  Wh. 

White. 

C. 

Cas. 

Cape. 
Castle. 

Cath. 

Cathedral. 

C.G. 

Coast  Guard 

Chy. 
161 

Chimney. 

M 

AIR    NAVIGATION    FOR    FLIGHT   OFFICERS 


Conspic. 

Conspicuous. 

L*  Ho. 

Lighthouse. 

Cr. 

Creek. 

V.  Vess. 

Light  Vessel. 

D. 

Doubtful. 

m. 

Mile-s. 

dist. 

Distant. 

Magz. 

Magazine. 

Estab*. 

Establishment. 

Mon*. 

Monument. 

Fm.,  Fms 

.Fathom-s. 

Mony. 

Monastery. 

F.S. 

Flagstaff. 

M'. 

Mountain. 

?,  ft. 

Foot,  Feet. 

Obsy. 

Observatory. 

F. 

Fort. 

Ord. 

Ordinary. 

h.,  hrs. 

Hour-s. 

Pass. 

Passage. 

Hd. 

Head. 

P.D. 

Position 

Hn. 

Haven. 

Doubtful. 

Ho. 

House. 

Penla. 

Peninsula. 

Hr. 

Harbour. 

Pk- 

Peak. 

L,  P. 

Island,  Islet. 

Posn. 

Position. 

Is. 

Islands. 

Promy. 

Promontory. 

in. 

Inch-es. 

R. 

River. 

L. 

Lake. 

Rf. 

Reef. 

Lit. 

Little. 

Rd.,Rds. 

Road-s. 

L,La,LagnLagoon.                R^R1*. 

Rock-s. 

Lat. 

Latitude. 

R.S. 

Rocket 

L.B. 

Life  Boat. 

Station. 

L.B.S. 

Life    Boat 

Ru. 

Ruin. 

Station. 

Ry. 

Railway. 

Ld*. 

Leading. 

s. 

Second-s. 

L6.,  L". 

Ledge-s. 

Sd. 

Sound. 

L.S.S. 

Life    Saving 

Sem. 

Semaphore. 

Station. 

Sh. 

Shoal. 

162 

SYSTEM  OF  LIGHTS 


Sig. 

Signal. 

Uncov. 

Uncovers. 

Stn. 

Station. 

Vil. 

Village. 

Str, 

Strait. 

W.T. 

Wireless    Tele- 

Tel. 

Telegraph. 

graphy  Stn. 

Temp7. 

Temporary. 

Yds 

Yard-s. 

Tr. 

Tree. 

System  of  Lighting 

Lights  may  be  divided  into  two  classes,  as 
follows : 

(1)  Those  whose  colour  does  not  change  in 
its  entire  system. 

(2)  Those  whose  colour  does  change. 
The    following    table    gives    the    various 

descriptions  of  the  different  lights. 


Lights    whise    Colo-n 
does  not  Change. 

Fixed 
Flashing 


Group  Flash- 
ing 


Characteristic  Phase. 

A  continuous  steady  light 

(1)  A  single  flash    at    regular 
intervals,  the  period  of  light 
being  less  than  the  period  of 
darkness 

(2)  A  steady  light    varied    at 
regular    intervals    with    a 
sudden  and  total  eclipse,  of 
greater  duration  than  the 
light 

Shows  a  group  of  two  or  more 
flashes  at  regular  intervals 

163 


Lights   whose    Colout 
does  Change. 

Alternating 

Alternating 

Flashing 


Alternating 
Group 
Flashing 


AIR  NAVIGATION    FOR   FLIGHT  OFFICERS 


Lights    whose    Colour 
does  not  Change. 

Occulting 


Group  Occult- 
ing 

Fixed        and 
Flashing 


Fixed         and 
Group  Flash- 
ing 

Revolving 


Characteristic  Phase. 

A  steady  light  varied  at  --regu- 
lar  intervals  by  a  sudden 
and  total  eclipse,  the  period 
of  light  being  equal  to  or 
greater  than  the  period  of 
darkness 

A  steady  light  varied  at  regu- 
lar intervals  by  a  group  of 
two  or  more  occultations 

A  steady  light  varied  at  regu- 
lar intervals  by  a  single  flash 
of  relatively  greater  brilli- 
ancy :  this  flash  may  or 
may  not  be  preceded  by  a 
short  eclipse 

As  above,  but  with  a  group  of 
two  or  more  flashes 


Light  gradually  increasing  to 
full,  then  decreasing  to 
eclipse 


Lights    whose    Colour 
does  Change. 

Alternating 
Occulting 


Alternating 
Group    Oc- 
culting 

Alternating 
Fixed  and 
Flashing 


Alternating 
Fixed  and 
Group 
Flashing 

Alternating 
Revolving 


The  letter  (U)  against  a  light  denotes  that 
it  is  unwatched,  and  too  much  reliance  must 
not,  therefore,  be  placed  on  seeing  it. 

Certain  details  of  the  lights  are  given 
opposite  them  on  the  charts  ;  should  a  fuller 
description  be  required,  all  details  will  be 
found  in  the  Admiralty  Light  Lists,  which 
are  published  every  year. 

The  height  stated  against  a  light  is  the 
height  of  the  centre  of  the  lantern  above 
high  water  springs. 

164 


LIGHT  VESSELS 

The  distance  of  visibility  given  in  the  light 
lists  and  against  the  light  on  the  chart,  is 
calculated  for  a  height  of  eye  of  15  feet  above 
the  sea  level. 

Light  vessels  are  painted  red  in  England 
and  Scotland,  and  black  in  Ireland,  with  their 
name  in  white  letters  on  each  side.  These 
latter  are  not  shown  during  the  war.  They 
carry  a  distinguishing  mark  by  day,  and  their 
light  by  night. 

Should  they  be  out  of  position,  they 
strike  their  day  mark  by  day  ;  and  at  night, 
instead  of  showing  their  light,  they  show  a 
red  light  at  each  end  of  the  vessel,  and  a  red 
flare  up. 

The  following  problems  all  come  under  the 
heading  of  chart  work,  and  will  be  found 
useful  at  times. 

To  Construct  a  Scale  of  Longitude  if  none 
is  given  on  Chart  : 

Draw  a  straight  line  AB  and  divide  it  into 
a  number  of  convenient  units  according  to  the 
scale  of  latitude  of  the  chart. 

From  the  point  A,  draw  a  line  AC  making 
with  the  line  AB  an  angle  BAG  equal  to  the 
latitude  of  the  place. 

165 


AIR   NAVIGATION  FOR   FLIGHT   OFFICERS 

From  each  of  the  divisions  a,  b,  c,  d,  etc.,  on 
the  line  AB,  draw  perpendiculars  to  the  line 
AC,  cutting  it  at  the  points  a',  b',  c',  d',  etc. 

The  divisions  Aa',  a'b',  b'c',  c'd',  etc.,  will 
be  the  scale  of  longitude  required. 


a       b       c       d      e       f 


Since  the  triangle  aAa'  is  a  right-angled 
triangle,  having  its  right  angle  at  a',  the  scale 
of  longitude  can  be  found  as  follows  : 

Aa'      r     •          A    / 
-  =  Cosine  aAa  . 

Aa 

i.e.  Aa'=  Aa  x  Cosine  aAa' 
or  scale   of  longitude  =  scale  of  latitude   x 
cosine  latitude. 

To  Lay  Off  a  Course.— This  is  a  compara- 
166 


LAYING  OFF  A  COURSE 

lively  easy  matter,  and  can  be  done  in  two 
ways. 

(a)  By  Parallel  Ruler. 

Join  the  points  of  departure  and  arrival 
by  a  straight  line.  Place  the  parallel  ruler  on 
this  straight  line,  and  transfer  its  direction 
to  one  of  the  compasses  engraved  on  the  chart 
so  that  the  edge  of  the  ruler  is  over  the  centre 
of  the  compass. 

The  reading  on  the  outer  edge  of  the 
compass  card  will  give  the  course  to  be 
steered. 

Care  should  be  taken  to  take  the  side  of 
the  compass  card  nearest  to  the  point  of 
arrival. 

(b)  By  Transparent  Protractor. 

Place  the  centre  of  the  protractor  on  the 
point  of  departure,  taking  care  that  its  sides 
are  pointing  true  north  and  south. 

Draw  the  string  tightly  along  until  it  is 
over  the  point  of  arrival. 

The  degree  on  the  protractor  over  which 
the  string  passes  will  be  the  true  course  to  be 
steered. 

Variation  must  be  applied  if  the  magnetic 
course  is  required. 

To  Allow  for  Drift  Due  to  Wind. — It  must 
167 


AIR  NAVIGATION  FOR   FLIGHT    OFFICERS 

be  remembered  that  the  compass  only  gives 
the  direction  of  the  machine  through  the  air, 
and  to  get  the  direction  of  the  actual  course 
made  good  over  the  land,  an  allowance  for  drift 
will  have  to  be  made. 

The  direction  of  this  allowance  must,  of 
course,  be  always  into  the  wind,  the  amount 
depending  on  the  speeds  of  the  machine  and 
wind,  and  the  relative  angle  between  the 
course  of  the  aeroplane  and  the  direction  of 
the  wind. 

The  method  of  finding  the  allowance  for 
the  drift  is  as  follows  : 

Example  : 

It  is  required  to  fly  from  A  to  B.  The  wind  is 
blowing  in  the  direction  shown  by  the  arrow  at 
10  units  (miles,  knots,  kilometres,  etc.)  per  hour. 
The  speed  of  the  machine  is  86  units  per  hour.  What 
is  the  course  to  steer,  and  what  will  be  the  distance 
made  good  over  the  land  in  one  hour  ? 

Join  AB. 

From  A  lay  off  a  line  AC  parallel  to,  and  with 
the  wind's  direction,  and  mark  off  along  it  a  distance 
AC  equal  to,  say,  one  hour's  effect,  i.e.  10  units. 

With  centre  C  and  radius  equal  to  86  units 
(i  hour's  machine  speed),  sweep  an  arc  cutting 
AB  at  D. 

168 


INTERCEPTING   HOSTILE   AIRCRAFT 

Join  CD  and  draw  AE  parallel  to  CD. 

AE  referred  to  the  compass  is  the  course  to  steer, 
and  AD  is  the  distance  in  units  made  good  over  the 
land  in  one  hour. 


FIG.  70. 

Intercepting  Hostile  Aircraft. — Three  cases 
come  under  this  heading  as  follows  : 

(a)  When  the  enemy  is  in  sight  of  the  pilot. 

(b)  When  they  are  out  of  sight  of  one 
another,  but  in  a  wind  of  the  same  direction 
and  strength. 

(c)  When  they  are   out  of  sight  of  one 
another,  and  in  winds  of  different  direction 
and  strength. 

N.B. — In  case  (c)  it  is  presumed  that  the 
force  and  direction  of  the  wind  at  the  place 

169 


AIR  NAVIGATION   FOR  FLIGHT  OFFICERS 

where  the  enemy  passed  over  has  been  tele- 
phoned to  the  air  station. 

Case  (a). — When  the  pilot  is  in  sight  of 
the  enemy. 

Upon  all  these  occasions  endeavour  to 
steer  a  converging  course  whilst  keeping  the 
compass  bearing  of  the  enemy  constant.  By 
doing  this,  you  are  approaching  him  in  the 
quickest  possible  way. 

If  observation  shows  that  the  compass 
bearing  of  the  enemy  is  changing  towards  the 
nose  of  your  machine,  it  means  that  he  will 
pass  ahead  of  you.  If  the  compass  bearing 
changes  towards  the  tail  of  your  machine,  it 
means  that  he  will  pass  behind  you. 

In  the  first  case,  the  course  should  be  altered 
away  from  the  enemy  ;  and  in  the  second  case, 
the  course  should  be  altered  towards  him. 

This,  of  course,  is  only  the  principle  of  the 
problem ;  the  two  machines  may  be  flying  at 
different  altitudes,  one  may  be  faster  than  the 
other,  the  enemy  may  alter  course,  etc.,  so 
that  much  must  be  left  to  the  pilot's  discre- 
tion ;  but  if  he  acts  on  the  above  principle,  he 
will  be  doing  all  he  can  to  close  the  enemy. 

Example : 

A  pilot  at  A  sights  an  enemy  machine  at  B, 
170 


INTERCEPTING   HOSTILE    AIRCRAFT 

bearing   100°,  and    steering    approximately  in  the 
direction  BM. 

A  steers  in  the  direction  AC  at  first,  and  on 


100' 


FIG.  71. 

arriving  at  C,  finds  the  bearing  of  the  enemy  machine 
to  be  still  100°.     He  therefore  keeps  on  his  course. 

At  E  he  finds  the  bearing  of  the  enemy  to  be  92°, 
showing  him  that  he  is  going  ahead  of  the  enemy. 

He  therefore  alters  his  course  to  EG. 
171 


AIR  NAVIGATION  FOR   FLIGHT  OFFICERS 

At  G  he  finds  that  the  enemy  is  bearing  100°, 
showing  him  that  the  latter  is  going  ahead  of  him. 

He  then  alters  course  to  the  direction  GJ,  and 
on  arrival  at  J,  finds  the  bearing  is  now  95°. 

He  then  steers  the  course  JL,  and  finding  that 
the  bearing  remains  constant  at  95°,  knows  that 
he  is  closing  as  fast  as  possible. 

Case  (b). — When  they  are  out  of  sight  of 
one  another,  but  in  a  wind  of  the  same  direc- 
tion and  strength. 

This  is  quite  a  simple  problem,  as  both 
being  affected  by  the  same  wind  force,  the 
latter  may  be  neglected,  and  the  only  thing 
to  do  is  to  consider  it  as  a  case  of  closing 
preserving  the  bearing. 

In  the  figure,  C  is  the  position  of  the 
enemy  when  reported,  and  A  the  aerodrome 
you  are  stationed  at,  situated  east  60  miles 
from  the  former.  He  is  reported  as  steering 
north  at  45  miles  an  hour,  and  the  speed  of 
your  machine  is  85  miles  an  hour. 

Firstly.  —  To  find  the  course  necessary 
to  steer,  the  following  procedure  should  be 
adopted. 

Join  AC. 

From  C  lay  off  the  enemy's  course  CE, 
and  mark  off  along  this  line  a  part  CB  equal  to 
the  enemy's  speed  for  one  hour,  i.e.  45  miles. 

172 


INTERCEPTING   HOSTILE   AIRCRAFT 

With  centre  B  and  a  radius  equal  to  your 
speed  for  one  hour,  i.e.  85  miles,  describe  an 
arc  cutting  CA,  produced  if  necessary,  at  D. 

Join  BD. 


FIG.  72. 


BD  will  be  the  course  to  steer  from  A. 

Secondly. — To  find  the  rate  of  closing,  or, 
in  other  words,  to  find  how  long  you  will  be 
before  you  will  catch  him. 

Measure  the  number  of  units  contained  in 


AIR  NAVIGATION   FOR   FLIGHT   OFFICERS 

the  line  CD.     This  will  be  the  rate  of  closing 
in  one  hour,  and  in  this  case  is  71  units. 

So  that  the  time  taken  to  catch  the  enemy 
will  be  : 

A  /""*  /" 

-~=r .     This  equals     ?  =  0*84  of  an  hour 

or  50*4  minutes. 

N.  B. — This  problem  can  be  worked  out 
either  on  a  chart  or  on  a  mooring  board, 
whichever  is  found  most  convenient. 

Case  (c). — -When  they  are  out  of  sight  of 
one  another,  and  flying  in  winds  of  different 
strength. 

Example. — Information  is  received  at  your  aero- 
drome that  a  hostile  machine  has  passed  over  a 
station  A,  making  good  290°  at  the  rate  of  40  miles 
per  hour. 

Your  aerodrome  is  190°  42  miles  from  this  point, 
and  you  have  a  machine  capable  of  a  speed  of  70 
miles  per  hour.  The  wind  at  your  station  is  north- 
east (45°)  at  12  miles  per  hour. 

What  course  must  you  steer  to  intercept  the 
enemy,  and  how  long  will  you  be  getting  there  ? 

Note. — It  should  be  remembered  that  the  enemy,  as 
reported,  is  making  good  course  and  speed  given.  If 
his  course  and  speed  and  direction  and  force  of  the 
wind  are  signalled,  you  will  have  to  work  out  first 

174 


INTERCEPTING   HOSTILE   AIRCRAFT 

what  he  is  making  good,  and  then  proceed  as  given 
below. 

A  is  the  position  of  the  enemy,  and  B  your 
aerodrome.     From  A  lay  off  AC  equal  to  one 


Scale 
0 


30 


GOMiles 


FIG.  73. 

hour's  course  and  distance  made  good  by  the 
enemy. 

From  C  lay  off  CD  in  the  direction  the 
wind  is  coming  from,  equal  to  the  speed  of 
your  wind  for  one  hour. 

With  centre  D  and  a  radius  equal  to  one 


AIR  NAVIGATION    FOR  FLIGHT  OFFICERS 

hour's  speed  of  your  machine,  sweep  an  arc 
cutting  AB,  produced  if  necessary,  at  E. 

Join  DE  and  EC. 

From  B  draw  BF  parallel  to  EC. 

DE  will  be  the  course  to  steer,  and  EC 
will  be  the  course  and  distance  made  good 
by  your  machine  in  one  hour. 

BF  is  the  course  and  distance  made  good 
by  steering  a  course  parallel  to  DE,  and  the 
two  machines  will  meet  at  F.  The  time  taken 

AF  .      32 

will  be  ~-T(^'  i.e.~—  =  0*8  hrs.  or  54  nuns. 
40 


These  distances  can  be  actually  measured 
on  the  chart  or  mooring  board,  and  the  time 
ascertained  from  that. 


176 


CHAPTER  XII 
FIXING  POSITIONS 

IN  an  aeroplane,  one  of  the  best  methods  of 
fixing  one's  position  is  to  be  able  to  read  a 
chart  or  map  thoroughly  so  that,  if  flying  over 
the  land,  one  can  tell  just  what  spot  is  verti- 
cally under  the  machine. 

As,  however,  this  is  not  always  possible  in 
a  seaplane,  it  is  proposed  to  explain  one  or 
two  methods  of  fixing.  The  last  method 
given  will  be  more  suitable  for  airships  or 
observation  balloons,  where  there  is  a  great 
deal  more  room  than  on  an  aeroplane. 

(a)  Fixing  by  '  Cross  Bearings.' — Choose 
two  objects  that  are  marked  on  the  chart  as 
nearly  90°  apart  as  possible,  as  this  will  give 
a  very  definite  cut. 

Correct  these  bearings  for  variation  and 
deviation,  and  you  are  ready  to  lay  off  on  the 
chart. 

Place  the  parallel  rulers  over  the  centre  of 
the  compass  engraved  on  the  chart,  and  turn  it 

177  N 


AIR  NAVIGATION    FOR   FLIGHT  OFFICERS 

round  until  the  edge  of  the  ruler  is  cutting  the 
corrected  bearing  on  the  edge  of  the  compass. 

Transfer  this  line  to  the  first  object,  and 
draw  a  line  through  it  in  the  opposite  direction 
to  your  bearing.  Do  exactly  the  same  with 
your  second  bearing. 

The  intersection  on  the  chart  of  these 
two  lines  will  be  your  position. 

Example : 


FIG.  74. 

After  correction  the  church  bears  336°  true  and 
the  flagstaff  57°  true. 

Draw  your  lines  in  the  direction  156°  and  237°^ 
i.e.  from  the  objects,  and  the  intersection  at  the 
circle  will  be  your  fix. 

(b)  Fixing  by  Doubling  the  Angle  on  the 
178 


FIXING  POSITIONS 

Bow.' — This  is  a  very  simple  method,  and 
merely  consists  of  taking  a  bearing  of  an 
object '  x  '  degrees  on  the  bow  of  your  machine 
and  noting  the  time,  and  again  taking  the 
bearing  when  it  is  '  2  x°  '  on  the  bow  with,  of 
course,  the  time  again.  Knowing  your  engine 
speed,  or  your  speed  over  the  land,  you  get 
your  distance  run  in  the  interval  of  time 
between  the  two  bearings  and  : 

Distance  run  in  the  interval  =  Distance  off 
at  second  bearing. 

Example : 


Course  East 


'True) 


FIG.    75. 

Speed,  60  miles  per  hour.     Course,  east  (true). 
9  A.M.     Tower  bore,  54°  (true). 
9.10  A.M.    Tower  bore,  27°  (true). 
Distance  run  in  10  minutes  is  10  miles. 
Therefore  position   at  second   bearing  is,  with 
tower  bearing  27°  (true),  distant  10  miles. 

179 


AIR  NAVIGATION  FOR    FLIGHT  OFFICERS 

Fixing  by  Station  Pointer. — To  understand 
this  method  of  fixing,  it  will  be  necessary  to 
go  into  the  theory  a  little. 

Fixing  by  station  pointer  does  not  call 
for  the  use  of  a  compass  :  all  that  is  required 
is  a  sextant  and  an  instrument  known  as  a 
station  pointer. 

The  station  pointer  fix  depends  on  a 
certain  theorem  in  Euclid  (iv.  5),  which  states 
that  a  circle  can  be  drawn  through  any  three 
points. 

If,  therefore,  three  points  on  the  chart  be 
chosen,  and  taking  our  position  as  the  fourth 
point,  it  is  obvious  that  we  can  draw  two 
circles  as  follows  : 

One  circle  passing  through  the  left-hand 
object,  the  middle  object,  and  our  position. 

The  other  circle  passing  through  the  right- 
hand  object,  the  middle  object,  and  our 
position. 

From  this  we  see  that  these  two  circles  will 
intersect  at  two  common  points,  viz.  at  the 
centre  object  and  at  our  position,  and  as  we 
cannot  be  at  the  former,  the  second  inter- 
section must  be  our  fix. 

Another  theorem  that  the  station  pointer 
fix  depends  on  is  Euclid  (in.  21),  which  states 
that  the  angles  on  the  circumference  of  a 

180 


STATION  POINTER  FIXES 

circle,  subtended  by  the  same  chord,  and  on 
the  same  side  of  the  chord,  are  equal  to  one 
another.  So  that  all  we  have  to  do  is  to  ob- 
serve two  angles  to  our  three  chosen  objects, 
and  place  these  angles  on  the  station  pointer 
and  fit  them  in  on  the  chart.  This  does  away 
with  the  necessity  of  actually  drawing  in  the 
circles.  The  size  of  the  circles  is,  of  course, 
governed  by  the  dimensions  of  the  observed 
angles. 

The  following  figure  shows  why  we  must 
be  at  the  second  intersection  of  the  two  circles. 


FIG.  76. 

The  angles  ADB  and  BDC  are  the  angles 
actually  observed.  Now  D  is  the  only  point 
we  can  be  at,  for,  supposing  we  were  at  E, 

181 


AIR  NAVIGATION    FOR   FLIGHT  OFFICERS 

although  the  angle  AEB  is  equal  to  the  angle 
ADB,  yet  the  angle  BEC  is  not  equal  to  the 
angle  BDC,  which  latter  was  the  one  taken  with 
the  sextant.  Hence  there  can  be  only  one 
place  that  will  fit  in  with  our  observed  angles, 
and  that  is  the  point  D  which  is  common  to 
both  circles. 

In  practice,  all  that  has  to  be  done  is  to  take 
two  angles  between  the  three  objects  chosen, 
place  these  angles  on  the  station  pointer,  fit 
its  three  legs  over  the  three  points  on  the 
chart,  and  the  small  nick  in  the  centre  leg 
indicates  your  position. 

A  certain  amount  of  care  is  necessary  in 
the  selection  of  the  objects.  The  following 
examples  are  worth  remembering  : 

(i)  The  objects  may  lie  in  the  same 
straight  line. 


STATION  POINTER  FIXES 

(2)  The  objects  may  lie  in  a  curve,  with 
the  middle  object  nearest  to  the  observer. 


FIG.  78. 


(3)  The  objects  may  lie  in  a  curve,  concave 
to  the  observer,  provided  the  latter  is  on  or 
within  a  line  joining  the  right  and  left  hand 
objects. 


AIR  NAVIGATION    FOR  FLIGHT  OFFICERS 

(4)  The  objects  may  lie  in  a  curve,  concave 
to  the  observer,  provided  the  latter  is  well 
outside  the  circle  passing  through  the  three 
objects. 


FIG.  80. 


(5)  Two  of  the  objects  may  be  in  transit 
with  the  observer.  In  this  case  one  angle  to  the 
third  object  is  all  that  it  is  necessary  to  take. 


STATION  POINTER  FIXES 

(6)  If  two  of  the  objects  are  much  nearer 
to  the  observer  than  the  third,  and  seem  about 
equidistant  from  the  observer,  at  whose 
position  they  subtend  an  angle  of  between 
60°  and  120°,  the  fix  is  a  good  one. 


FIG.  82. 


185 


CHAPTER  XIII 
ORDNANCE  MAPS 

THESE  maps  are  to  the  pilot  flying  over  the 
land,  what  a  chart  is  to  a  seaman  navigating  a 
ship,  with  the  advantage  that,  given  a  clear 
day,  the  pilot  can  always  see  the  land  below 
him,  which  is  impossible  in  a  ship. 

There  is  a  much  greater  wealth  of  detail, 
as  regards  the  land,  in  an  ordnance  map 
than  in  a  chart,  as  obviously  a  navigator  at 
sea  does  not  require  the  topography  for  any 
distance  inland. 

Ordnance  maps  are  constructed  on  the 
gnomonic  projection,  and  are  not  provided 
with  any  magnetic  compass,  so  that  all 
courses  have  to  be  referred  to  the  true  north 
and  south,  which  direction  is  given  on  the 
inner  border  of  the  map.  The  sides  of  an 
ordnance  map  are  not  graduated  like  a 
Mercator's  chart,  but  a  scale  is  provided  at 
the  bottom  of  each  map  in  whatever  unit  of 
length  it  is  drawn  to,  i.e.  miles,  yards,  or  feet. 

186 


MARKINGS  ON  ORDNANCE  MAPS 

The  statute  mile  of  5280  feet  is  used  in 
ordnance  maps,  unlike  the  Admiralty  chart, 
where  the  unit  is  a  sea  mile. 

In  using  an  ordnance  map  which  is  not 
squared,  it  is  convenient  to  draw  a  series 
of  parallel  lines  to  the  true  north  and  south 
lines  to  facilitate  laying  of  courses.  It  is  also 
better  to  cross  these  lines  with  east  and  west 
ones. 

Conventional  Markings. — These  are  as 
follows  : 

Hills  are  shown  in  brown,  their  heights 
being  those  above  a  certain  level,  which  is 
given  at  the  foot  of  each  map. 

Rivers  and  canals  are  coloured  blue. 

First  class  roads  are  coloured  red. 

Second  class  roads  are  left  uncoloured. 

Railways  are  denoted  by  thick  black 
lines. 

If  the  map  shows  any  part  of  the  sea,  this 
is  coloured  blue. 

Towns  and  villages  are  represented  by 
black  blocks  of  rectangular  or  other  shape t 
with  the  streets  running  through  them,  the 
amount  of  detail  shown  depending  on  the 
scale  of  the  map. 

Woods  are  coloured  green. 
187 


AIR  NAVIGATION    FOR    FLIGHT  OFFICERS 

Lakes  are  coloured  blue. 

Isolated  houses  are  denoted  by  black 
dots. 

All  other  symbols  conform  to  those  given 
on  Admiralty  charts. 

To  Lay  Off  a  Course. — A  celluloid  pro- 
tractor is  supplied,  marked  from  o  to  360  in 
the  same  way  as  a  compass  card. 

It  is  pierced  in  the  centre,  and  a  string  is 
let  through  the  hole. 

To  lay  off  a  course,  the  centre  of  the 
protractor  is  placed  on  the  starting-point,  with 
its  sides  parallel  to  the  true  north  and  south 
line. 

Place  the  string  over  the  point  it  is  desired 
to  go  to,  and  draw  it  tight. 

The  degree  on  the  protractor  that  the 
string  passes  over  will  be  the  true  course 
required. 

To  Measure  a  Distance. — This  is  done  by 
means  of  a  pair  of  dividers.  Place  one  point 
of  the  latter  on  the  starting-place  and  the 
other  point  on  the  place  you  wish  to  go  to. 
Transfer  this  distance  to  the  scale  at  the 
bottom  of  the  map. 

Should  the  distance  be  too  big  for 
the  dividers,  put  a  certain  or  convenient 

188 


SQUARED  MAPS 

distance  on  the  latter  from  the  scale,  and  run 
this  distance  along  a  straight  line  joining  the 
two  points,  noting  how  many  times  it  goes 
into  the  total  distance. 

Squared  Maps. — These  are  ordnance  maps 
divided  into  large  rectangles,  each  named  by 
a  letter. 

These  rectangles  are  divided  into  thirty 
or  thirty-six  squares,  each  of  whose  sides  are 
1000  yards  long. 

The  squares  are  numbered  from  i  to  30, 
or  i  to  36,  starting  at  the  top  left-hand  corner 
and  running  across  to  the  right. 

Each  of  these  squares  is  divided  into  four 
squares,  each  of  whose  sides  is  500  yards  long. 

These  squares  are  lettered  as  shown 
below. 


FIG.  83. 


In  any  report  sent  in,  the  centre  of  the 

189 


AIR  NAVIGATION  FOR  FLIGHT  OFFICERS 


small  square  is  taken  as  the  spot  mentioned  ; 
but  if  more  accuracy  is  required,  a  cardinal 
or  semi-cardinal  point  can  be  introduced, 
giving  the  direction  of  the  object  from  the 
centre  of  the  small  square. 

If  great  accuracy  is  required,  each  side  of 
the  small  square  can  be  divided  into  ten  equal 
parts,  each  50  yards  long,  always  starting  from 
the  south-western  end  of  the  square.  In  this 
case,  the  number  along  the  east  and  west  line 
is  always  mentioned  first. 

Example  : 


FIG.  84. 

Supposing  there  was  a  windmill  in  square  '  c  ' 
as  shown,  and  it  was  required  to  report  its  position 
accurately. 

It  was  located  in  rectangle  B.  26  c.,  but  to  be 
absolutely  accurate,  it  should  be  reported  as  Wind- 
jnill  B.  26  c.  2,  4. 

190 


SQUARED  MAPS 

A  sketch  of  the  whole  rectangle  is  given 
below. 


1 

2 

3 

4 

5 

a       b 

6 
c       d 

7 

8 

9 

10 

II 

12 

13 

14 

15 

r 

16 
> 

17 

18 

19 

20 

L 

21 

> 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

FIG.  85. 

Only  one  of  the  squares  of  each  rectangle 
is  marked  a,  b,  c,  d.  This  is  to  prevent  over- 
crowding, but  all  the  others  follow  the 
same  law. 

A  scale  is  provided  at  the  bottom  of  each 
squared  map,  and  a  magnetic  compass  is 

191 


AIR  NAVIGATION    FOR   FLIGHT    OFFICERS 

printed  on  the  north-west  corner.  The  topo- 
graphy is  the  same  as  on  an  ordinary  ordnance 
map. 

Selection    of    Suitable   Landmarks,    etc. — 

When  flying  from  one  place  to  another,  it  is 
desirable  to  check  the  position  as  frequently 
as  possible. 

This  can  easily  be  done  in  clear  weather, 
provided  the  pilot  can  read  his  map 
thoroughly. 

If  possible  before  a  flight,  the  pilot  should 
look  over  his  map,  and  note  what  he  would 
expect  to  pass  over  on  his  way.  During  the 
flight  he  should  endeavour  to  pick  up  each  of 
these  marks  as  he  passes  them. 

Roads,  rivers,  canals,  railways,  bridges, 
lakes,  woods,  villages,  and  towns  are  all  good 
marks,  as  are  tall  chimneys,  churches,  clumps 
of  trees  on  hills,  etc. 

Very  often  a  distant  mountain  peak  or 
other  conspicuous  object  will  give  him  a  good 
mark  for  direction,  either  by  steering  straight 
for  it,  or  keeping  it  a  little  on  one  side  of  the 
machine. 


192 


APPENDIX 


Variation  of  Wind  Velocity  with  Height,  page  206. 
The  Gradient  Wind,  page  208. 


These  Tables  have  been   included  here  by  the  kind 
permission     of  the  National    Physical   Laboratory, 
Teddington,  Middlesex. 


AIR  NAVIGATION    FOR   FLIGHT    OFFICERS 

printed  on  the  north-west  corner.  The  topo- 
graphy is  the  same  as  on  an  ordinary  ordnance 
map. 


these  marks  as  he  passes  them. 

Roads,  rivers,  canals,  railways,  bridges, 
lakes,  woods,  villages,  and  towns  are  all  good 
marks,  as  are  tall  chimneys,  churches,  clumps 
of  trees  on  hills,  etc. 

Very  often  a  distant  mountain  peak  or 
other  conspicuous  object  will  give  him  a  good 
mark  for  direction,  either  by  steering  straight 
for  it,  or  keeping  it  a  little  on  one  side  of  the 
machine. 


192 


APPENDIX 


.     315 
No  Devn. 


225 

No Devn 


135 

No  Devn. 


Ely  Devrv. 
180  Max. 


315 

No Devn 


225 

No  Devn 


135 

NoOevn. 


Wly  Devn. 
180  Max. 


Coefficient  E 


APPENDIX 

Coefficient  E. — This  is  due  to  the  effect  of  induc- 
tion in  horizontal  soft  iron  running  diagonally. 

If  it  runs  from  left  front  to  right  rear,  '  E  '  is  -f-  ; 
if  from  right  front  to  left  rear,  '  E  '  is  — 

It  is  maximum  on  the  cardinal  points,  diminish- 
ing to  zero  on  the  quadrantal  points. 

It  is  found  by  taking  the  mean  of  the  deviations 
on  the  cardinal  points,  changing  the  signs  of  those 
on  east  and  west. 

It  is  called  '  Quadrantal '  because  it  changes  its 
sign  in  each  quadrant. 

It  is  corrected  in  conjunction  with  Coefficient 
'  D  '  by  placing  the  spheres  at  an  angle  6  with  the 
transverse  line,  if  '  D '  is  -f  ;  and  with  the  longitudinal 
line,  if  '  D  '  is  — ;  so  that 

jr 
Tangent  26  =— *  • 

If  '  E  '  is  4-  the  left-hand  sphere  goes  in  front : 
vice  versa  for  —  E. 

The  amount  to  be  corrected  is 


+  E2 

See  Diagram  on  opposite  page. 

Composition   of   the   Air. — Air    is    an    invisible 
193 


APPENDIX 

gas  largely  composed  of  nitrogen  and  oxygen  in 
the  following  proportions  : 

Nitrogen    ....  77*11  per  cent. 

Oxygen      ....  20-65 

Water  Vapour    .          .          .  1-40 

Argon         ....  0'79 

Carbonic  Acid     .          .          .  0-04 

Height  of  the  Air. — From  various  observations, 
the  most  important  of  which  is  that  of  meteors,  it 
is  estimated  that  the  major  portion  of  the  atmos- 
phere extends  about  one  hundred  miles  above  the 
earth's  surface,  also  that  it  exists  from  there  to  a 
height  of  400-500  miles,  but  of  course  in  a  very  much 
thinner  form. 

Density  of  the  Air. — The  atmosphere  is  densest 
at  the  surface  of  the  earth,  and  gets  gradually 
more  and  more  attenuated  until  its  confines  are 
reached.  At  a  height  of  about  seven  miles  it  has 
only  one-quarter  of  the  surface  density ;  about 
fourteen  miles,  one-sixteenth ;  whilst  at  twenty-one 
miles  merely  one-sixtieth. 

The  Meteorological  Elements. — Under  this  head- 
ing come  the  following  : 

Pressure,  Temperature,  Humidity,  Wind,  and 
Cloud.  The  last  has  already  been  dealt  with  in  the 
body  of  the  book,  and  wind  partly  dealt  with. 

(i)  Pressure. — By  this  is  meant  the  capability 
of  the  density  of  the  air  at  sea  level  to  support  a 
column  of  mercury  enclosed  in  a  glass  tube. 

This  pressure  is  nearly  always  changing,  hence 
the  reading  of  the  barometer  scale  indicating  the 

194 


APPENDIX 

height  of  this  column  is  scarcely  ever  the  same  from 
hour  to  hour. 

Pressure  is  measured  by  a  barometer,  which 
is  merely  a  glass  tube  filled  with  mercury,  which 
is  then  boiled  to  expel  any  particles  of  air  or 
water  vapour,  and  then  inverted  into  a  cup 
mercury. 

The  mercury  will  fall  in  the  tube  until  the  pres- 
sure of  the  outside  air  balances  its  fall  and  prevents 
any  further  drop  in  the  tube.  The  space  between 
the  top  of  the  enclosed  column  of  mercury  and  the 
top  of  the  tube  is  the  nearest  known  approach  to  a 
perfect  vacuum,  and  is  known  as  a  '  Torricellian 
Vacuum/ 

If  now  the  pressure  of  the  air  increases,  it  will 
press  more  heavily  on  the  mercury  in  the  cup.  This 
will  be  communicated  to  the  mercurial  column, 
causing  it  to  rise  in  the  tube.  Conversely,  if  the 
atmospheric  pressure  decreases,  it  will,  by  not  press- 
ing so  heavily,  cause  the  column  to  fall  in  the  tube. 
This  is  known  as  the  rise  or  fall  of  the  barometer, 
and  its  amount  is  measured  by  a  fixed  and  also  a 
movable  scale  at  the  side  of  the  tube ;  the  latter  is 
known  as  the  Vernier. 

Owing  to  friction  at  the  sides  of  the  glass  tube, 
the  top  of  the  mercury  assumes  a  convex  form,  as 
shown  below. 

When  reading  the  barometer,  the  bottom  of 
the  pointer  of  the  vernier  plate  should  be  brought 
down  by  the  milled  screw  at  the  side  so  as  to 
touch  the  top  of  the  mercury,  as  seen  in  the  sketch. 

Owing  to  what  is  known  as  the  '  Vertical  Pres- 
sure Gradient/  barometer  readings,  when  sending  in 

195 


APPENDIX 


reports,   are  corrected  for  their  height  above  sea 
level  to  reduce  them  to  the  latter,  this  being  the 


common  level  used  on  meteorological  charts.     The 
barometer  has  also  to  be  corrected  for  the  tempera- 


-Vernier  Plate 
(graduated) 


^^  Pointer  of 
Vernier  Plate 


APPENDIX 

ture,  owing  to  the  column  of  mercury  in  the  tube 
expanding  or  contracting  according  to  the  rise  or 
fall  of  the  temperature. 

(2)  Temperature  is  the  thermal  condition   of    a 
body  which  determines  the  exchange  of  heat  be- 
tween it  and  some  other  substance.     Heat  may  be 
imparted  in  three  ways  : 

(i)  Radiation.     (2)  Conduction.     (3)  Convection. 

(3)  Humidity.  —  Interspersed  between  the  mole- 
cules of  nitrogen  and  oxygen,  which  are  the  chief 
constituents   of   air,    are   also   molecules   of  water 
vapour  invisible  because  of  their  transparency. 

This  water  vapour  is  caused  by  the  continued 
evaporation  which  is  always  taking  place  from 
water,  ice,  snow,  or  any  moist  surface.  This  quantity 
of  water  vapour  is  constantly  changing  owing  to 
the  evaporation  from  the  earth's  surface  becoming 
faster  or  slower.  As  the  temperature  rises,  the 
capacity  of  dry  air  for  holding  moisture  increases, 
so  that  the  warmer  the  air,  the  greater  quantity  of 
water  vapour  it  can  sustain  in  an  invisible  state. 
Now  any  given  volume  of  dry  air  can  only  take  up 
a  certain  invisible  quantity  of  water  vapour,  and 
when  this  amount  is  exceeded  the  latter  becomes 
visible  as  cloud  mist  or  fog.  The  humidity,  or 
in  other  words,  the  amount  of  moisture  in  the  air, 
can  be  gauged  by  means  of  the  wet  and  dry  bulb 
thermometer. 

Heating  and  Cooling  of  the  Atmosphere. — The  air 
receives  its  heat  from  the  sun,  but  being  a  bad 

197 


APPENDIX 

conductor,  only  gets  a  very  little  of  it  by  conduction. 
The  sun's  rays  pass  through  the  air  and  strike  the 
earth,  the  amount  of  heat  the  latter  received  de- 
pending on  the  obliquity  of  the  rays.  The  earth 
radiates  this  heat  received,  which  warms  the  layer 
of  air  in  immediate  contact  with  it ;  this  warm  air 
rises  and  cold  air  fills  its  place.  This  latter  is  known 
as  convection,  so  that  the  air  is  chiefly  warmed 
by  radiation  and  convection  and  only  slightly  by 
conduction. 

Measurement  of  Temperature. — Temperature  is 
measured  by  means  of  a  thermometer,  an  instru- 
ment consisting  of  a  glass  bulb  and  tube,  the  latter 
partly  filled  with  mercury  or  alcohol,  the  latter  for 
use  in  very  cold  climates.  In  graduating  the  ther- 
mometer we  know  of  two  fixed  points  which  are 
always  the  same  at  sea  level,  viz.  the  boiling  and 
freezing  points  of  distilled  water. 

The  thermometer  being  placed  in  each,  marks 
are  made  showing  the  level  which  the  mercury 
attains,  and  the  space  between  is  divided  into  a 
convenient  number  of  divisions  called  degrees. 

Two  kinds  of  thermometers  are  in  use : 
The  Fahrenheit  thermometer. 
The  Centigrade  thermometer. 

(i)  Fahrenheit  Thermometer. — The  boiling  and 
freezing  points  having  been  marked,  the  space 
between  them  is  divided  into  180  equal  parts. 

When  this  thermometer  was  first  invented,  it 
was  also  put  into  a  mixture  of  ice  and  salt,  which 
produced  the  lowest  known  cold  in  those  days.  The 

198 


APPENDIX 

point  to  which  the  mercury  descended  was  taken 
as  the  zero  of  the  scale,  and  was  thirty- two  divisions 
below  the  freezing  point  of  distilled  water.  Hence, 
in  a  Fahrenheit  thermometer,  freezing  point  is 
represented  by  32°  and  boiling  point  by  212°. 

(2)  Centigrade  Thermometer. — In  this  thermo- 
meter the  space  between  the  freezing  and  boiling 
points  of  distilled  water  is  divided  into  100  parts;  so 
that  freezing  point  is  represented  by  o°  and  boiling 
point  by  100°. 

The  Absolute  Zero. — By  this  is  meant  the  tempera- 
ture at  which  gases  would  have  no  volume  and  exert 
no  pressure  if  they  went  on  contracting  with  cooling 
as  at  ordinary  temperatures. 

This  temperature  is  about  459°  below  zero  of 
Fahrenheit. 

Measurement  of  Pressure. — Pressure  is  measured 
by  the  barometer  or  aneroid,  whose  scale  is  marked 
in  inches  or  millibars. 

The  latter  is  about  the  thousandth  part  of  the 
ordinary  atmospheric  pressure  at  sea  level,  and  is 
also  known  as  a  '  pressure  limit/ 

A  table  giving  the  equivalents  of  mercury  inches, 
millimetres,  and  millibars  is  given  on  p.  9  of  the 
'  Handbook  of  Meteorology.'  29-92  mercury  inches, 
which  is  the  normal  pressure  in  the  British  Islands  = 
1013-2  millibars ;  10  millibars  =  0-03  mercury 
inches. 

The  Vertical  Pressure  Gradient. — This  is  the 
decrease  in  the  height  of  the  mercury  in  the  baro- 

199 


APPENDIX 

meter  owing  to  the  rarefied  air  being  unable  to 
support  the  same  column  that  it  could  on  the  sea 
level.  This  fall  amounts  to  i  inch  of  mercury  in 
about  900  feet. 

Deflection  of  Wind  due  to  the  Earth's  Rotation.— 
The  maximum  velocity  of  the  rotary  motion  of  the 
earth  occurs  at  the  equator  and  diminishes  to  zero 
at  either  pole. 

In  consequence  of  this,  a  mass  of  air  flowing  from 
a  high  to  a  lower  latitude,  i.e.  towards  the  equator, 
will  be  deflected  to  the  westward,  owing  to  the 
increased  velocity  of  the  earth.  On  the  other  hand, 
a  mass  of  air  flowing  from  a  low  to  a  higher  latitude, 
will  be  deflected  to  the  eastward,  owing  to  the  earth's 
decreasing  velocity. 

For  example,  a  southerly  wind  in  the  Northern 
Hemisphere,  i.e.  a  wind  blowing  from  the  equator 
towards  the  pole,  will  be  deflected  to  the  right  and 
becomes  south-westerly  ;  and  a  northerly  wind,  i.e. 
setting  from  the  pole  towards  the  equator,  is  also 
deflected  to  the  right  and  becomes  north-easterly. 

The  direction  right  or  left  is  obtained  by  standing 
with  your  back  to  the  wind. 

The  reverse  holds  good  in  the  Southern  Hemi- 
sphere, the  northerly  wind  being  deflected  to  the 
left  and  becoming  north-westerly,  and  the  southerly 
wind  being  deflected  to  the  left  and  becoming 
south-easterly. 

From  this  we  see  that  when  an  air  current  sets 
towards  an  area  of  low  pressure,  from  the  surround- 
ing high  pressure,  it  is  deflected  to  the  right  and  left 

200 


APPENDIX 


Example  : 

(i)  Northern  Hemisphere. 
N 


S  S 

FIG. 88. — Northern  Hemisphere.  FIG.  89. — Northern  Hemisphere. 
Southerly  wind.  Northerly  wind. 

(2)  Southern  Hemisphere. 
N 


S 

FIG. 90. — Southern  Hemisphere.   FIG. 91. — Southern  Hemisphere. 
Southerly  wind.  Northerly  wind. 

in  the  Northern  and  Southern  Hemispheres  respec- 
tively. This  air  current  does  not  set  directly  towards 
the  low  pressure,  but  acquires  a  motion  round  it, 
but  inclined  inwards  towards  the  centre  of  the  low 

201 


APPENDIX 

pressure.  This  circular  motion  is  against  the  hands 
of  a  watch  in  the  Northern  Hemisphere,  and  with 
the  hands  of  a  watch  in  the  Southern  Hemisphere. 

Again,  when  the  air  from  an  area  of  high  pressure 
flows  towards  an  area  of  low  pressure,  it  is  deflected 
to  the  right  or  left  according  to  its  hemisphere,  and 
acquires  a  motion  round  the  high  pressure  area 
inclined  outwards. 

This  motion  is  with  clock  hands  in  the  Northern 
Hemisphere,  and  against  clock  hands  in  the  Southern 
Hemisphere. 

The  Different  Forms  of  Isobars. — Isobars  are 
divided  into  seven  different  groups,  of  which  the 
cyclonic  and  anti-cyclonic  types  have  already  been 
given ;  the  remainder,  together  with  the  weather 
encountered  in  them,  are  given  below. 

(3)  Secondary  Cyclone. 

Deiacned  Cloud 
29  90 


3000 

Cirrus 


FIG.  92. 

A  secondary  cyclone  is  usually  found  on  the 
edge  of  a  cyclone,  but  very  often  on  that  of  an 
anti-cyclone. 

202 


APPENDIX 


(4)  The  Wedge. 


Cyclone 


29-50 


29-80 


3010 


30  30 

FIG.  93- 
(5)  The  Straight  Isobar. 

29  50 

29-60_     Cold  Ram 
29-70 


APPENDIX 

The  wedge  is  an  area  of  high  pressure  interposed 
between  two  cyclonic  depressions. 

The  gradients  are  slight  and  the  wind  never 
strong. 

The  isobars  may  run  in  any  direction.  The 
wind  is  generally  strong  or  gusty  but  does  not 
attain  gale  force. 

(6)  The  V  Depression. 


FIG.  95. 

The  point  of  the  V  is  generally  directed  towards 
the  equator,  and  in  the  Northern  Hemisphere  the 
convex  side  of  the  trough  is  usually  facing  to  the 
eastward. 

204 


APPENDIX 

The  wind  does  not  veer  in  the  usual  manner, 
but  the  passage  of  the  trough  is  marked  by  a  sudden 
shift  of  wind  and  a  violent  squall. 

(7)  The  Col. 

This  is  an  area  of  low  pressure  between  two 

areas  of  high  pressure.  No  typical  weather  is  met 

with,  but  the  presence  of  a  col  indicates  unsettled 
conditions. 

N.B. — Arrows  show  direction  air  currents  are 
flowing  towards. 

Land^and  Sea  Breezes. — These  are  met  with  in 
the  tropics  and  also  in  the  temperate  regions  during 
fine  settled  weather. 

They  are  caused  by  the  unequal  heating  of  land 
and  water. 

After  sunrise  the  land  gets  heated  quicker  than 
the  sea,  consequently  the  air  above  the  former  rises, 
and  the  cool  air  over  the  latter  flows  in  to  take  its 
place,  causing  the  '  sea  breeze/ 

After  sunset,  the  land  parts  with  its  heat  quicker 
than  the  sea,  so  that  the  warm  air  above  the  latter 
rises  and  the  cooler  air  from  the  land  flows  out  to 
take  its  place,  causing  the  '  land  breeze/ 

Variation  of  Wind  Velocity  with  Height. — It  has 
been  found  by  experiment  that  the  velocity  of  the 
wind  increases  with  the  height,  and  tends  to 
gradually  become  parallel  to  the  isobars. 

The  veering  of  the  wind  with  height  may  be 
roughly  estimated  in  degrees  from  a  certain  formula. 

205 


APPENDIX 

Where  '  V  '  is  the  veering  and  '  H  '  the  height  : 

30  X  -^L 

y  _  IOQO 

H 


1000 

Height.  Veering. 

O  0° 

IOOO  10° 

2OOO  15° 

3000  18° 

4000  20° 

5OOO  21° 

600O  22^° 

7000  23^° 

8000  24°J 

9000  24^° 

10000  25° 

i 1000  25^° 

12000  25!° 

Fluctuation  and  Gustiness. — The  velocity  of  the 
wind  is  seldom  uniform,  but  varies  in  gusts  and  lulls. 

The  difference  between  the  average  maximum 
velocity  of  the  gusts  and  the  average  minimum 
velocity  of  the  lulls  is  known  as  the  '  fluctuation 
of  the  wind/ 

The  gustiness  of  the  wind  is  found  as  follows  : 

Fluctuation 

Gustiness  =  -  — - — — 

Average  velocity 

Let  V  be  the  maximum  and  v  the  minimum 
velocity. 

V-  v 
Then  Gustiness  =  — 

V  +  v 


206 


APPENDIX 

It  has  been  found  that  the  gustiness  of  the 
wind  at  any  particular  place  for  a  given  direction  is 
practically  constant. 

Twilight. — This  is  caused  by  the  air  reflecting  a 
certain  quantity  of  light  from  the  sun  when  the 
latter  is  below  the  horizon. 

Thus,  twilight  occurs  twice  a  day,  in  the  morning 
and  evening.  There  are  two  kinds  of  twilight — 
Astronomical  and  Civil.  Astronomical  twilight 
begins  and  ends  when  the  sun's  centre  is  18°  below 
the  horizon,  when  only  first  magnitude  stars  are 
visible.  It  will  last  all  night  if  the  latitude  and 
declination  are  of  the  same  name,  and  their  sum  is 
not  less  than  72°. 

Civil  twilight  begins  and  ends  when  the  sun's 
centre  is  6°  below  the  horizon,  when  stars  of  the  first 
magnitude  are  not  visible. 


The  Gradient  Wind. — Observation  has  shown 
that  a  wind  due  to  a  difference  of  pressure  between 
two  places  is  greater  the  bigger  the  difference  of 
pressure  and  the  closer  the  isobars. 

If  the  differences  of  pressure  over  a  certain  area 
are  marked  on  a  chart  by  means  of  isobars,  it  is 
possible  to  calculate  the  force  of  the  wind  by  means 
of  a  formula. 

This  wind  is  known  as  the  '  Gradient  Wind/ 
but  the  formula  does  not  take  friction  into  account. 
The  gradient  direction  should  be  regarded  as  along 
the  isobars. 

207  p 


APPENDIX 

The  following  was  table  issued  for  finding  the 
gradient  velocity  of  the  wind  :  * 

V  is  the  velocity  of  the  wind,  and  D  the  distance 
apart  of  the  isobars  in  nautical  miles. 

Then  V  =  2        at  a  height  of  about  2000  feet, 

where  D  is  the  distance  between  the  isobars  corres- 
ponding to  YQ  of  an  inch  of  mercury. 

D  j          v 

280  I0  4200 

140  20  V  =  ~"rj~  when  D  =  distance 

100  28  J-.-L.--L. 

apart   of  the     isobars   corres- 
ponding to  0-5  of  a  centibar. 

If  the  standard  distance  apart  of  the  isobars, 
i.e.  fifteen  miles,  is  used,  the  following  table  gives 
the  velocity  of  the  gradient  wind,  assuming  ordinary 
conditions  of  pressure  and  temperature  and  making 
no  allowance  for  the  curvature  of  the  path : 

Barometric  Pressure  Difference  VpWitv 

per  15  Nautical  Miles. 

O'Oi  inch  19  miles  per  hour. 

0'02     ,,  39 

0-03     „  58      „ 

0-04     „  77      „ 

0-05     „  97      ,, 

The  observed  velocity  is  seldom  the  same  as  the 
theoretical  velocity,  the  latter  being  usually  con- 
siderably in  excess  of  the  former. 

*  Permission  to  reprint  above  table  has  been  given  by  the 
Controller  of  H.M.  Stationery  Office. 

208 


APPENDIX 


High  and  Low  Pressure  Areas. — On  account  of 
the  circulation  of  the  air,  the  latter  in  high  latitudes 
is  moving  faster  than  the  earth's  surface.  This 
increases  its  centrifugal  force,  making  it  press  on 
the  air  in  low  latitudes.  The  expansion  of  the  air 
over  the  tropics,  due  to  the  heat,  causes  it  to  press 
on  that  in  higher  latitudes. 

This  combined  effect  causes  a  distribution  of 
pressure  as  shown  roughly  in  the  figure  below. 


High 


Low 


High 


FIG.  96. 

The  land  being  more  quickly  affected  by  change 
of  temperature  than  water,  bigger  changes  are 
experienced,  due  to  change  of  seasons,  on  land  than 
on  sea. 

Triangle  of  Velocities. — Velocity  not  only  signi- 
fies the  rate  of  pace  but  embraces  the  quarter  from 
which  any  force  travels.  The  velocity  of  a  point 
may  be  represented  by  a  straight  line,  the  speed  being 

209 


APPENDIX 

measured  by  scale,  and  the  direction  it  is  moving 
in  by  the  direction  of  the  line. 

If  a  point  A  has  two  velocities  AB  and  AC,  the 
resultant  velocity  is  represented  by  the  line  AD, 
which  is  the  diagonal  of  the  parallelogram  ABCD. 

In  practice,  it  is  only  usual  to  draw  the  two  lines 
AC  and  CD,  of  which  the  third  side  AD  is  the 
resultant. 

Example : 


FIG.  97. 


Speed  of  Machine 
,.  Wind 


North,  60  knots. 
N.E.,  20  knots. 


Required,  resultant  velocity  both  in  magnitude 
and  direction.  Scale  10  knots  =  i  inch  (see  Fig.  98). 

Radius  of  Action. — The  radius  of  action  in  a 
particular  direction  is  the  farthest  distance  in  that 
direction  that  a  machine  can  go  and  return.  The 
area  of  action  is  the  area  to  every  point  of  the  peri- 
meter of  which  an  aeroplane  can  j  ust  go  and  return. 

Distance  =  Speed  X  time 

0  Distance 

Speed  = 


Time  = 


Time 

Distance 

Speed 


210 


APPENDIX 


Example : 


Then 


FIG.  98. 

Wind  ahead,  20  knots. 
Speed  of  machine,  80  knots. 
Fuel  for  5  hours. 

Let  R  =  Radius  of  action. 

RT~> 

I 

60       ioo 

R  -  187-5. 


A  machine  flies  i  mile  in  60  sees,  with  wind, 
i.e.  60  M.P.H. 

A  machine  flies  i  mile  in  ioo  sees,  against  wind, 
i.e.  36  M.P.H. 

211 


APPENDIX 

Find  its  speed  in  still  air. 

Let  V  and  v  be  velocities  in  still  air  and  in  the 
wind  respectively. 

V  +  v  =  60 
V  -  v  =  36 

Then  2V  =  96 

V  =  48  M.P.H. 

Radii  of  Action. — These  can  be  worked  out 
graphically,  knowing  radius  of  action  with  and 
against  the  wind  together  with  fuel  hours,  and  the 
results  plotted  on  squared  paper,  the  resulting 
radii  being  afterwards  drawn  in. 


212 


INDEX 


The  letter    '/'  after  a  page  number  indicates  '  following  page 
or  pages.' 

PAGE 

ABBREVIATIONS,  general      ...  .      i6if. 

light  ....                              .  .      161 

tidal  and  buoy     .  .160 

ABRIDGED  NAUTICAL  ALMANAC    .         .         .  125,  136 

ABSOLUTE  ZERO 199 

ACTION  OF  AN  AEROPLANE,  area  and  radius  of  .      2ioff. 
ADJUSTMENT  OF  DEVIATION         ....        32ff« 

ADMIRALTY  CHARTS               .  7»  I43f» 

fathom  lines  used  on     .          .  .      I58f. 

light  lists     ...  .164 

'  Manual  of  Navigation  '  •  80,  83 

signs  and  symbols  used  on    .  .      154^- 
variation  chart     ......          7 

AERO  COMPASS  .         .  •       ^f. 

allowance  for  heeling    .  .        29 

essential  features  in  an  .        28f. 

expansion  and  contraction    .  .        29 

lighting  of  ...  .29 

making  of  the  card 29! 

steadiness    ...                    •  .28 

AIR,  composition  of    .  •      J93f- 

density  of   .  •      J94 

height  of  the                                       .  *94 

humidity  in  the  .  •      197 

pressure  of  the     .                    •  • 

AIRCRAFT,  intercepting  hostile      .          .  • 

213 


INDEX 

PAGE 

ALLOY  USED  IN  MAKING  MAGNETS         ...  14 
AMPLITUDE  TABLES    .         .          .          .          .         .138 

ANALYSIS  OF  DEVIATION     ....         32ff.,  4 if. 

ANGLE  OF  DEVIATION 8 

hour             .......  121 

of  variation          .          .          .          .          .          .  yf. 

to  find  hour          ......  i4of. 

ANTI-CYCLONE    .......  7 iff. 

APPARENT  DEVIATION 33 

AQUILA 99 

AREA  OF  ACTION  OF  AN  AEROPLANE     .          .          .  2ioff. 

AREAS,  high  and  low  pressure      ....  209 

ARIES         ........  99 

ARTIFICIAL  MAGNETS           .....  2 

ASCENSION,  right        .          .          .          .          .          .113 

ASTRONOMICAL  TIME 115 

twilight       .......  207 

ASTRONOMY,  notes  on          .....  93ff- 

ATMOSPHERE,  heating  and  cooling  of  the       .          .  i97f. 

AURIGA     ........  100 

Axis  OF  THE  EARTH no 

AZIMUTH  TABLES        ......  i27f. 

BANKING  ON  A  COMPASS,  effect  of         .          .          .31 

BAROMETRIC  SURGE 71 

BEARING  AMPLITUDE  .          .          .          .          .138 

rule  for  naming  the      .          .          .          .          .137 
BEARINGS,  swinging  a  compass  by  reciprocal          .        45 

fixing  position  by  cross          ....      i77f- 

taken  from  the  tables,  true  ....        62ff. 
BEAUFORT'S  SCALE  FOR  SEA  DISTURBANCE    .         .        85 

system  of  weather  notation  ....        84f. 

system  of  wind  notation        .          .          .          .        83f. 

BLOCK,  magnet .31 

BOOTES     ........      100 

BREEZES,  land  and  sea 205 

BRITISH  ISLES,  weather  in  the     ....       8off. 
'  BROADSIDE-ON  '  MAGNET  .....       2off. 

214 


INDEX 


PAGE 


BUBBLE  FROM  COMPASS,  removal  of  .  .26 
BUOY  ABBREVIATIONS  .  .  .  .  .160 
BURDWOOD,  DAVIS  AND  .  .  .  .  127,  1 2Q 
BUYS-BALLOT'S  LAW 69 

CANIS  MAJOR     .         .         .          .  .         .      101 

CANIS  MINOR      .......      101 

CASSIOPEIA          ......          96,  98 

CATUS .          .      102  ; 

CELESTIAL  CONCAVE    .         .          .          .  in,  114 

equator        .          .          .          .          .          .          .112 

CENTIGRADE  THERMOMETER         ....      199 

CENTRE  OF  A  STORM  ......        68 

CHART,  gnomonic        ......      i43f. 

Mercator's  .......      145^- 

CHARTS,  Admiralty     .          .          .          .          .  7,  143$. 

CIRCLES,  definition  of  great  and  small  .          .      no 

of  declination       .          .          .          .          .          .112 

CIRCUMPOLAR  CONSTELLATIONS    ....       95 

CIVIL  TIME         .         .         .         .          .          .          .113 

twilight        .......      207 

CLOUD,  formation  of  a  .          .          .          .        736°. 

CLOUDS,  composite      ......        75 

fundamental         ......        74 

COEFFICIENTS  OF  DEVIATION       ,.          .          .     32ff.,  193 
COL  ISOBAR,  the  ......      205 

COMPASS.     See  Aero  Compass. 

correction  of  a  ....       43rf.,  54ff. 

effect  of  banking  on  a  .          .          .          .31 

error  ........          9 

essential  features  in  an  aero  .          .  28f. 

landing        .......        45f. 

liquid  used  in  a   .          .          .          .          .          .25 

magnetic     .......        23ff. 

methods  of  swinging 43ff. 

needle,  earth's  effect  on         ....          4f. 

placing  a     .          .          .          .          .          .          .27 

removal  of  bubble  from         .          .          .          .26 

215 


INDEX 

PAGE 

COMPASS,  shore  .......       45!. 

to  test  a  .          .          .          .          .          .66 

'  COMPASSES  FOR  USE  IN  AIRCRAFT  '     .          .          -31 

COMPOSITE  CLOUDS 75 

COMPOSITION  OF  THE  AIR  .....      193!". 
CONCAVE,  celestial       .          .          .          .          .       in,  114 
CONDUCTION  OF  HEAT         .....      198 
CONSTANT  DEVIATION  33 

CONSTELLATIONS,  circumpolar      ....        95 
CONTRACTION  IN  AERO  COMPASS  ...        29 

CONVECTION  OF  HEAT 198 

COOLING  OF  THE  ATMOSPHERE     ....      igjf. 

CORONA  BOREALIS       ......      102 

CORRECTION  OF  A  COMPASS  .          .          .       43^.,  54ff. 

of  courses    .......        57ff. 

CORVUS .          .      107 

COURSE,  to  lay  off  a  .          .          .          .          .        167,  1 88 

COURSES,  correction  of        .....        57!?. 

CRUX         ........      103 

CYCLONES  .......        68 

secondary    .......      202 

CYGNUS 103 

DAVIS  AND  BURDWOOD       ....      127,  129 

DECLINATION,  circles  of  .          .          .         .112 

parallels  of  .          .         .          .          .          .112 

DEFLECTION    OF    WIND    DUE    TO    THE    EARTH'S 

ROTATION 2ooff. 

DENSITY  OF  THE  AIR          .....      194 

DEVIATION,  angle  of  .          .          .          .          .         .         8 

analysis  and  adjustment  of  .          .          .        32ff.,  4if . 
apparent  or  constant    .          .          .          .          -33 

coefficients  of 32fL,  193 

effect  of  vertical  iron  on        ....       4of. 
how  to  name        ......        ^gfi. 

natural 55 

quadrantal            .          .          .          .          .  16,  38 

semicircular 16,  34,  36 

2l6 


INDEX 

PAGE 

DEVIATIONS,  analysis  of  a  table  of        .         .         .       4 if. 
DIP  MAGNETIC  ANGLE  .         .         .         •    5,  9 

DISTANCE,  to  measure          .         .         .         .      151,  188 
DIVIDED  TOUCH  IN  MAKING  MAGNETS  .         .        12 

DRIFT  DUE  TO  WIND,  to  allow  for        .         .         .      1671. 
DUNBOYNE'S  WEATHER  REPORT  ....       ySf. 

EARTH,  axis  of  the     .         .          .         .          .          .no 

effect  on  compass  needle  ^f. 

poles  of  the          .          .          .          .          .          .no 

ECLIPTIC  PATH 112 

ELECTRO  MAGNET  IN  MAKING  MAGNETS,  use  of       .        13 
ELEMENTS,  meteorological  ....  194-9 

'  END-ON  '  MAGNET     .....         i8f.,  22 

ENGLISH  CHANNEL,  weather  in     .          .          .          .        8 if. 

EQUATION  OF  TIME     .         .         .          .          .117,  1231. 

EQUATOR  .          .          .          .          .          .          .          .no 

equinoctial  or  celestial  .          .          .          .112 

magnetic     .......          8 

EQUINOCTIAL  POINTS  .          .          .          .          .112 

ERROR,  compass          ......          9 

EXPANSION  IN  AERO  COMPASS     ....        29 

FAHRENHEIT  THERMOMETER         ....      ig8i. 

FATHOM  LINES  USED  ON  ADMIRALTY  CHARTS          .      158! 

FITZROY'S  WEATHER  RULES,  ADMIRAL  .          .         . 

FLYING  GROUND,  methods  of  marking  out  a          . 

swinging  a  compass  by  marked -out        .          .        47 

FOG,  cause  of     .          .          .          .          .          .          -75 

FORCE,  horizontal       ......          9 

lines  of        ......  2f.,  8 

method  of  denoting  wind       ....        77 

vertical        .......        10 

FORECASTING  OF  WEATHER  ...  76,  86ff. 

FORMATION  OF  CLOUD,  FOG,  AND  DEW          .         .        73:8:. 

FUNDAMENTAL  CLOUDS 74 

GEMINI       ........      104 

GEOGRAPHICAL  POLES 2 

217 


INDEX 


GNOMONIC  CHART        .         .          .          .          .  143^ 

G.M.T ii8f. 

GRADIENT,  vertical  pressure          .          .          .     i95f.,  1991. 
wind  ........      207f. 

GREENWICH  OBSERVATORY  .         .         .         .      n8f. 

'  HANDBOOK  OF  METEOROLOGY  '  .          .          .199 

HAVERSINE  TABLE       .         .         .          .          .          .127 

HEAT,  radiation,  conduction  and  convection  of      .      197!. 
HEATING  OF  THE  ATMOSPHERE    ....      i97f. 

HEAVENS,  poles  of  the          .          .          .          .  in 

HEIGHT  OF  THE  AIR  .....      194 

variation  of  wind  velocity  with     .          .          .      205ff. 
HIGH  PRESSURE  AREAS       .  209 

HORIZONTAL  FORCE  .....         9 

HOSTILE  AIRCRAFT,  intercepting  ....      i69ff. 
HOUR  ANGLE      .         .         .          .          .         .          .121 

to  find          .......      i4of. 

HUMIDITY  IN  THE  AIR        .....      197 

INMAN'S  TABLES          ....      i26f.,  138,  i48ff. 

IRIDIUM  POINTS,  use  of                  .          .          .  .24 

IRON  ON  DEVIATION,  effect  of  vertical            .  .        4of. 

effect  of  magnetism  on  hard  and  soft     .  .        15! 

ISOBAR  LINE      ......  69,  77 

ISOBARS,  different  forms  of           ...  202-5 

ISOTHERMAL  LINE       .....  69,  77 

LAND  AND  SEA  BREEZES    .....     205 

LANDING  COMPASS      ......       45f. 

LANDMARKS,  selection  of  suitable          .          .          .192 
LATITUDE,  magnetic  ......         9 

of  a  place    .          .          .          .          .  .in 

parallels  of in 

LAW  OF  MAGNETISM,  first  ....         3 

LEO 104 

LIGHT  ABBREVIATIONS 161 

2l8 


INDEX 


PAGE 


LIGHTING  OF  AERO  COMPASS         ....         29 

system  of    .          .          .          .          .          .          .      i63f. 

LINES  OF  FORCE  .....  2f.,  8 

LIQUID  USED  IN  A  COMPASS          .          .  .25 

LOCAL  TIME       .          .          .          .          .          .          .119 

LODESTONE         .......          i 

LONGITUDE  OF  A  PLACE       .          .          .          .  in 

on  time,  effect  of  .          .          .          .          .117 

to  construct  a  scale  of  ....      i6$i 

Low  PRESSURE  AREAS        .....      209 

LYRA 105 

MAGNET  BLOCK  .         .         .         .          .          .31 

'  broadside-on '     .          .          .          .          .          .        2off . 

electro          .          .          .          .          .          .          .13 

'  end-on '  .          .          .          .         i8f.,  22 

MAGNETS,  artificial      ......          2 

divided  touch  in  making       .          .          .          .12 

effect  of  temperature  on  .          .  i5f. 

methods  of  making       .          .        , .          .          .        nf. 
natural         .......          if. 

percussion  in  making    .          .          .          .          .11 

single  touch  in  making  .          .          .          .        nf. 

MAGNETIC  ALLOY        .          .          .          .          .          .14 

compass       .......        23ff. 

dip  angle  .          .          .          .     5,  9 

equator        .......          8 

latitude 9 

meridian      .......          yf. 

poles  .          .          .          .          .          .          .      2,  8f. 

MAGNETISM,  elementary iff. 

first  law  of  ......          3 

on  hard  and  soft  iron,  effect  of  .          .        i5f. 

sub-permanent     ......        i6ff. 

'  MANUAL  OF  NAVIGATION,  '  ADMIRALTY  .          .80,  83 

MAPS,  Ordnance i86ff. 

use  of  squared      .  .  .  iSgff. 

MEAN  SOLAR  TIME      .         .         .          .         .         .116 

219 


INDEX 

PAGE 

MEASUREMENT  OF  PRESSURE  OF  AIR  .         .         .      199 
of  temperature     ......      198 

MEASUREMENTS,  distance     .          .          .          .       151,  1 88 

MERCATOR'S  CHART 145^- 

MERIDIAN,  magnetic  ......          yf. 

passage        .......      121 

prime .in 

MERIDIANS          .......      nof. 

standard 119 

METEOROLOGY   ......     67  ft.,  194-9 

'  METEOROLOGY,  HANDBOOK  OF  '          .          .         .199 
MOONRISE  AND  MOONSET,  how  to  find  time  of       .      i32ff. 

NATURAL  DEVIATION  .....       55 

magnets      .......          if. 

NAUTICAL  ALMANAC,  Abridged     .         .          .       125,  136 
'  NAUTICAL  ALMANAC  TABLES  '     ....      i23ff. 

NEEDLE  OF  COMPASS,  earth's  effect  on.          .          .       4f. 

ORDNANCE  MAPS         ......     i86ff. 

ORION  .  ....  97,  105,  109 

OSBORNE,  R.N.,  CAPTAIN  F.  CREAGH    .         .         .31 

PARALLELS  OF  DECLINATION         .          .          .          .112 

of  latitude  .          .          .          .          .          .  in 

PASSAGE,  meridian      .          .          .          .          .          .121 

PATH  OF  A  STORM 68 

PEGASUS  .          .          .          .          .          .98,  105 

PERCUSSION  IN  MAKING  MAGNETS          .          .          .11 
PIVOTS,  broken  ......        30 

POLAR  DISTANCE  OF  A  HEAVENLY  BODY      .          .113 

POLE  STAR 96 

POLES,  angle  of  variation  ....          7 

geographical         ......          2 

magnetic     .          .          .          .          .          .  2,  8f. 

of  the  earth          .          .          .          .          .          .no 

of  the  heavens     .          .          .          .          .          .in 

of  the  magnet      ......       2,Jtf. 

22O 


INDEX 


POSITION  BY  CROSS  BEARINGS,  fixing   .          .         . 

by  doubling  the  angle  on  the  bow  .          .179 

by  station  pointer         .....      iSoff. 
PRECESSION  OF  STARS          .....        95 
PRESSURE  AREAS,  high  and  low  .          .          .      209 

gradient,  vertical  ....      195!:.,  iggi. 

of  the  air    .......      194^. 

measurement  of,  of  the  air  ....      199 

PRIME  MERIDIAN         .          .          .          .          .          .      in 

PRISMS       ........        30 

PROCTER,  R.  A.  .          .          .          .          .          -93 

QUADRANTAL  DEVIATION       .  .  .  .    1  6,    38,   193 

RADIATION  OF  HEAT  .....  197 

RADIUS  OF  ACTION  OF  AN  AEROPLANE         .          .  2ioff 
REFLECTORS       .......        30 

RIGHT  ASCENSION       ......  113 

ROTATION  OF  STELLAR  SPHERE    ....        95 

RUBY  POINTS,  use  of  ......        24 

SAPPHIRE  POINTS,  use  of     .          .          .          .          .24 

SCORPIO     ........      107 

SEA  BREEZES     .......    ^205 

Disturbance  Scale,  Beaufort's         .          .          .     '  85 
nature  of     .          .          .          .          .          .          .      159 

SECONDARY  CYCLONES         .....      202 

SEMICIRCLES  OF  A  STORM    .....        68 

SEMICIRCULAR  DEVIATION   .          .          .          .    16,  34,  36 

SHORE  COMPASS          ......       45f. 

SIGNALS,  storm  .......        82f. 

SIGNS  USED  ON  ADMIRALTY  CHARTS     .          .          .      154*!. 
SINGLE  TOUCH  IN  MAKING  MAGNETS     .          .          .        nf. 
SOLAR  TIME,  apparent  .          .          .          .116 

mean  .  116 

SPHERE,  definition  of  a        .          .          .          .  no 

SQUARED  MAPS,  use  of         .....      iSgft. 

STANDARD  MERIDIANS         .         .         .          .         .119 

221 


INDEX 

PAGE 

STAR  PRECESSION        .          .          .          .  *       ,          .95 

swinging  a  compass  by  a  .          .          .44 

STAR  ATLAS,  PROCTER'S       .....        93^ 

STATION  POINTER,  fixing  position  by     .          .          .    iSoff. 
STELLAR  SPHERE,  rotation  of  .          .          -95 

STORM,  centre  of  a  .          .          .          .          .68 

path  of  a     .          .          .          .          .          .          .68 

semicircles  of  a    .          .          .          .          .          .68 

signals         .......        82f. 

trough  of  a  .          .          .          .          .          .68 

STRAIGHT  ISOBARS      ......      203 

SUB-PERMANENT  MAGNETISM         ....        i6ff. 

SUMMER  TIME 119 

SUN,  mean         .          .          .          .          .          .          .116 

swinging  a  compass  by  the   .          .          .          .44 

SUN'S  TRUE  BEARING  TABLES      ....      i2yf. 

SUNRISE  AND  SUNSET,  how  to  find  time  of     .          . 
SURGE  IN  THE  BAROMETER  .          .          .          .71 

SWINGING  A  COMPASS,  methods  of        ... 

by  distant  object  .          .          .          .          .46 

by  marked-out  flying  ground          .          .          .47 
by  reciprocal  bearings  .....        45 

by  sun  or  star      ......        44 

by  two  objects  in  line  .          .          .          .          .47 

SYMBOLS  USED  ON  ADMIRALTY  CHARTS          .          .      154*1. 

TABLE,  HAVERSINE      .          .          .          .          .          .127 

of  deviations,  analysis  of  a    .          .          .          .        4 if. 

TABLES,  amplitude      .          .          .          .          .          .138 

explanation  of  various  .          .          .          .      I23ff. 

Inman's i26f.,  138, 

sun's  true  bearing  or  Azimuth        .          . 

TAURUS 106 

TEMPERATURE    ...  ...      197 

measurement  of  .          .          .          .          .          .198 

on  magnets,  effect  of  .          .          .          .  i5f 

TESTING  A  COMPASS .66 

222 


INDEX 

PAGE 

THERMOMETERS ig8f. 

TIDAL  ABBREVIATIONS         .          .          .          .          .160 

TIME  AMPLITUDE 138 

astronomical         .          .          .          .          .          .115 

effect  of  longitude  on   .          .          .          .          .117 

equation  of  .          .          .          .          .117,  I23f. 

local  .          .          .          .          .          .          .          .119 

notes  on      .......      113^. 

solar  .          .          .          .          .          .          .          .116 

summer       .          .          .          .          .          .          .119 

TORRICELLIAN  VACUUM       .....      195 

TOUCH  IN  MAKING  MAGNETS,  single  and  divided  .  nf. 
TRIANGLE  OF  VELOCITIES  .....  209f. 

TROUGH  OF  A  STORM 68 

TRUE  BEARING  TABLES,  SUN'S  ....  i27f. 
TRUE  BEARINGS  TAKEN  FROM  THE  TABLES  .  .  62ff« 
TWILIGHT,  ASTRONOMICAL  AND  CIVIL  .  .  .  207 

UPPER  MERIDIAN  PASSAGE  .          .          .          .121 

URSA  MAJOR      ......         96,  108 

URSA  MINOR 96,  106 

V  DEPRESSION  ISOBAR  .....  204 
VACUUM,  TORRICELLIAN  .....  195 
VARIATION,  angle  of  (magnetic  and  geographical)  .  7! 

of  wind  velocity  with  height           .          .          .      2O5ff. 
VELOCITY  WITH  HEIGHT,  variation  of  wind           .      2O5fL 
VELOCITIES,  triangle  of       .....      209f. 
VERNIER  PLATE 196 

tube 195 

VERTICAL  FORCE         .          .          .          .          .          .10 

pressure  gradient  ....     i95f.,  i99f- 

WEATHER,  forecasting  of     ....  76,  86fL 

in  the  British  Isles Soff. 

in  the  English  Channel  .          .          .          .        8 if. 

method  of  denoting      .....        78 
notation,  Beaufort's  system  of       ...      84^ 

223  Q 


INDEX 

PAGE 

WEATHER  report,  Dunboyne's      ....        y8f. 

rules,  Admiral  Fitzroy's         ....        Sgff. 

WEDGE  ISOBAR  ......  203 

WIND,  cyclonic  .......        68 

due  to  the  earth's  rotation,  deflect icn  cf         .  2ooff. 

force,  method  of  denoting     .          .          .  77 

gradient      .          .          .          .          .          .  20  7f. 

notation,  Beaufort's  system  of       ...        83! 

to  allow  for  drift  due  to        ....  i67f. 

velocity       .          .          .          .          .          .          .  2  05  if. 

ZERO,  absolute  .         .         .         .         ,         .  199 


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